This is because the Wii hardware has a capacity that's quite inferior to modern graphics cards, and this can make all the difference in games with high definition (yes, Wii in HD). The best part of the emulator is that, in most cases (or at least if your computer is relatively new), the graphics will be even better than they are on the original consoles.
With it, you can play virtually any game from the Nintendo catalog with no problems.
Dolphin is the best emulator for Nintendo Wii and Gamecube currently available.