The final goal is indeed to use all letters. If you just go around choosing words, let's say, at random, you'll end up with places where you cannot use some letters. Then you will have to unmark some "wrong" (valid but not the ones in the searched solution) words and try to find some other combinations that don't leave unused cells.
Note that a "score" is calculated when you reach the 100% filled solution. This score penalizes marking "wrong" words and going back and unmarking them. So if you want to not only solve the puzzle but get a good score, you need to be careful not to go too fast adding just whatever word you find. Instead, try to build words which don't leave isolated letters, e.g. start in a corner and build from there, or from the bottom, etc.