What is the use of 5V pins on Raspberry Pi GPIO?
The Raspberry Pi board is powered off of 5V from the USB connector. The pins you see on the header can also power the Raspberry Pi in case you wanted to make a Hat that did something like provide battery power to the Pi. You could also make Hat's that use the 5V supply (sparingly) to power peripherals (like an Arduino Hat) and use level-shifters for communication back with the Pi.

5V is required for things like the HDMI, so it just makes sense to use 5V as the power input, and step it down to 3V3 for everything else. It also makes sense from a supply standpoint because phone chargers (that supply USB-5V) are cheap and in ready supply, so using one to power a super-cheap computer brings the total cost down to somebody who wants to get into it.

You can look at the Raspberry Pi "schematics" here although they are pretty reduced since they don't release full schematics.