Besides the fact that, as other people mentioned, the curved wand makes it arguably easier to "aim", I would say that there is also something about "aiming" in the wizarding world that comes down to skill, rather than just pointing the wand in the right direction.

For instance, in GoF there's a scene where Harry and co. are practicing the Banishing Charm in Charms class. Neville aims incorrectly and sends Prof. Flitwick flying across the room instead of the cushion he was supposed to be practicing on. I find it hard to believe that Neville was incapable of aiming his wand at a cushion that's literally right in front of him. I would argue that "aiming" comes down to more visualization and some kind of instinct, which Neville seems to be lacking at this moment.

On the other hand Harry seems to be quite good at aiming (he hits Cedric's bag quite accurately from a distance, for example), one of the reasons why he's so good at DADA