You can go into other universes once you buy a 5-D tangerine. Doing so allows you to access perks, and complete speed and universe achievements.
Doing such gets you 1 uCoin for every e5 tangerines gained (rounded down), so if you have gained 1e10 tangerines, you would get 2 uCoins, and if you gained 1e15 tangerines, you would get 3 uCoins and so on.