A tribe of indians in the Amazon thought the World was flat. But what kept the World up? Why didn't it fall down? Ah! Because it was laying on the back of a giant turtle of course!
But, some things, like the "universal" question, just can't be answered. I think that's why the concept of "god" comes in. The human brain needs reasoning behind everything. Otherwise we wouldn't go exploring the world we live in. JRRT tries to create a world of his own, but somehow he can't complete it. Maybe, it isn't for us to know everything. I don't know.
Sometimes the need for reasoning lead to a fear of an open question. Which can lead to people being satisfied by answers like "it's not for us to know", "it is just the way it is" or claims that the question cannot be answered. Then people don't dare to pry deeper, or to queston the "Obvious Truths" Then we get the spanish inquisition.
Clearly, if we approch ME as a fictional place in a book, then there is little point in this discussion. Frodo did what Frodo did because the author wrote it that way. End of story. Why did he write it that way? We can speculate, but the chap is dead. He can't tell us his motivations anymore.
If we look at ME as a "real" place, however, then this chatting of ours can indeed enlighten us a bit more. I think for Tolkien ME did become a real place in his mind. He didn't always agree with himself on all the details, but ME as a place was real.
Amarië said the a,b,c,d,e... theory was simpler than mine. Wrong you are! In order to use a,b,c to deside all the happenings and non-happenings in ME the a,b,c-plan would require a truly horrendous complexity! Mine doesn't require anything - exept that Eru can't be bound by time. If he created the world, time being part of the world, there is really no reason for him to be bound by time anyway. It is just so difficult for us to comprehend a state of being where time doesn't behave the way we are accustomed to.
As I by now is quite used to think in 4 dimentions I sometimes find it hard to understand the trouble others have when trying to do that. I do remember a time when I too found that to be the mother of headache-creaters though.
Why is time so difficult? Let's start with the other dimentions. Commonly denoted x,y and z. Lets choose, say, the North-South axis as the x-axis. Then the East-West direction (from the point you are in) is the y-axis and the z-axis is straight up and down. When standing up I span 1.8 meters in the z-direction and about 50 cm in the x and y directions. I am with other words in several different (but close) points in space at the same time. I am 5 cm above the ground, 50 cm and 150 cm above the ground at the same time!
With our eyes we can look in all the three directions. Even if we look just in one direction and blocks off the others with walls or something, we can see a lot of different points in that direction at the same time. Here. Over there. Behind there again. And Far Away. In fact we can see an amazing amount of points. All of which together makes up the fabric of space. The ground, the trees... When we can see all this in one glimpse we get what we call overview. We don't just see every point one after an other, that would be really confusing! We see them all at once, and how they fit together. The tree isolated can be a confusing thing. Why does it look like that? But when we see the tree in it's right context it makes sense. The roots spreads out and into the ground, seeking water. The branches go up and out, trying to get as much sunlight as possible.
Now consider an animated movie. It consists of drawings on flat sheets of paper. When we flip the pages fast enough, we see it as the figures on the paper moves around.
Unlike in space, where we occupy many points at the same time, we occupy only one point in time. That point is Now. Now isn't static, it changes. It moves. But it is still only one point. Unlike the other directions, where we could sit still, or move back and forth as much as we wants to and as fast or as slowly as we wants to, time always move in only one direction. It never goes backwards. And it always moves with constant speed.
Not only can we not occupy more than one point in time, but we can only see one point in time also! Like only seeing the place your feet are, never anywhere else. Would it be easy to make sense of our world if we only saw the ground beneath our toes? No way! Same with time. We are struggeling!
Back to the animation. Space is there the piece of paper. Time is the page number. Lets say page 5 is Now. The figure in the space (paper) can perhaps see the whole page. But it can't see page 6 or page 4. Perhaps it remembers stuff from page 4, and perhaps it has a plan for the future that might or might not be fulfilled on page 6, but it doesn't see either of them. We, however, can. We can take all the pages and lay them down side by side on a table. Now we can see page 1,2,3,4,6,7,8,9..... all in one glimpse! Because we are not in the space of the cartoon figure (the paper), nor in the time of the cartoon figure(the turning of pages) We are outside of all of that and that gives us powers that the 'toons don't have.
Same in ME. Eru created a world. He populated it. He let them (unlike in a cartoon made by humans) have their own free will. Regardless of their choises he can see all the pages at once. He therefore knows the result without having to intervene and "micromanage" his creatures. Maybe he "leaks" some of his knowledge to one of his creatures as a prophecy? Doesn't that mean that he then force the destiny of ,say Eowyn? No. He just already know what she will do, even before she knows it herself.
As for Tolkiens motivations... ME in general and Silmarillion especially is very influenced by norse mythology. And one thing is for sure: for the norsemen the world was totally deterministic. They believed strongly in fate and destiny. Maybe that means Tolkien wanted ME to be a deterministic place? Or maybe he gave his characters free will, but needed a bit of predestiny to give the right "feel" to it?