Well it just powers your system. The wall outlet puts out AC current, while computers need DC current the power supply converts the AC current into usable DC current. Problems can arise if the PSU is of bad quality and dies. There is a chance that along with it going the PSU could also take other more expensive components with it. If you don't change the computer at all you should be fine (changing it may void a warranty if there is one this is the reason for not changing it). But if you ever want to make changed, do replace the PSU.
Well since I don't know the exact model/specs of the PSU in the computer I can't say for sure about the efficiency. But it will most likely be better for either of the PSU I linked, I doubt the one in the computer is 80+ rated at all.