Reasons to turn a computer off when it isn't in use include decreasing the power bill and not wasting electricity by having it on. It is also a good idea to get better performance from the computer, as occasionally rebooting it can help it run more optimally.
Turning off the computer also ensures there aren't additional noises coming from the computer while sleeping. If it is a laptop that gets too hot when left on, the fan might power on and disrupt sleep. If the volume is left on, noise from open websites, pop-ups or notifications can also be disruptive.
On the other hand, some people like having updates run while sleeping so they don't deal with them while using the computer. This might be a good reason to keep a computer on overnight.