Question: I've just been wondering because I see this on ubiquity and want to know how this is better during the installation of Ubuntu.

Answer: 
If you are not plugged, then your laptop battery may be dead before the 
installation is finished, if you have a laptop. **;)**

The result can be a ruined or incomplete installation and you have to start over. Assuming your hard drive still functions.
There is another point. When you are on battery performance is all depend on battery but while charging performance increase little bit and laptop all depends on direct DC charge and it gives little bit more volts than battery.