the bottom line is, substrate bottoms make it easier for live food (e.g. blackworms) to hide and for uneaten food to go unnoticed and spoil the water. Bare bottom tanks make cleaning up much easier.
Alternatively, you can even have a planted tank with a soil-based substrate (search for "walstad method" or walstad tank), which is very pretty, but requires some attention to detil and careful water monitoring while the tank cycles (which is a good idea for any newly set up tank).
If you go bare bottom, you can still have live plants if you either use rootless plants or those that grow floating (e.g. hornwort) or plant your plants in shallow dishes which you set into the tank.