There are several reasons to install a new power supply.  Some computers have a power supply that does not produce enough power for the computer. Some computers come with power supplies that are fine right out of the box, but after several new drives and upgrades have been added it might not do the job it once did.  If this is the case then a more powerful power supply will need to be installed. Also, power supplies can easily fail. In such case a new power supply will also be necessary.