One of the most important mechanics almost all games have to get right is movement.
So many promising games in the past have been let down by the fact that their movement just isn't any good. You can have the best ideas in the world and it won't mean a thing if the player has no fun getting around the environments you've created.
It truly can make or break a game.
Look at something like Star Fox: Zero, for example. What could have been an enjoyable, if fairly unoriginal, game ended up being a complete trainwreck because the movement was so awful.
Although Star Fox: Zero is a particularly egregious example of poor movement in gaming, there are so many more like it that just don't put enough thought into what it'll be like for the player to play the game.
However, there are some games that don't settle at just getting their movement systems right. Some games go above and beyond in creating satisfying mobility, and end up making their games legendary because of it.
These games simply wouldn't be what they are without their movement systems, and that's a testament to how great they truly are.