There are a few benefits of using an amp stand.

Firstly the controls are at a better height to access, secondly you can generally hear the higher frequencies more and have a better indication of volume compared to the speakers pointed right at the back of your legs, and thirdly and probably the reason most people use them, is to remove the amp from direct contact from the floor as this can cause a "boominess" which enhances the bass frequencies and can make your amp sound muddy or muddle up the frequencies and make it sound messy.

Having said that. putting your amp on a table or chair, or milk crates or anything really that lifts it off the floor will give you the same effect, so try it with whatever you have at hand. Usually amp stands are tilted back a little so can get the same effect but only being a few inches off the floor.

Thanks for answering, Joliet.

You tell me that it does influence the sound a lot.

I´ll try it by lifting it off the floor, like you recommended, before buying anything...