For the most part covers are not needed unless you get something plastic that could save you while transporting the amp in rainy weather. They are so cheap it is a false economy not to use them. Particularly if you have a cat, they seem to love clawing speaker cloth. It stops dust getting in as well.
Moreover, how do you make an isolation box for a guitar amp?
Likewise, what are amplifiers wrapped in? The fabric covering that Fender uses today is not Tolex, but rather textured vinyl. It remains the most common form of covering for Fender amps; usually black but sometimes in other colors too.
Secondly, what is an amp cover for?
Amp covers not only keep your amplifier looking clean, but also prevent dust and debris from getting in the nooks and crannies.