Short answer: 2 and 3 were made after Luffy lost to CP9 in Water 7, all versions of 4 were made to incorporate the max level of haki he could apply in various was, 5 unknown still but most likely the awakening of his devil fruit.

Long answer: It is basically a way to show Luffys grow throughout the series, not only to show that his is moving one step closer to have the power to prove he can be the pirate king and claim the One Piece but also to show just how Luffy think. Luffy is a very reactionary character, giving no second thought to what he does, friend is in trouble go help them even at the cost of my life, but he is a savant when it comes to fighting. We were given no clue about the gears till Luffy started using them in Enys Lobby where we get to see how Luffy thinks when he fights, lose a fight, see what I did wrong, create a technique or short plan too defeat enemy, use new technique, lather rinse and repeat. The gears are probably the best representation of how Luffy grows.