Mountain Biking

If you are only just starting out I really don't feel like it is a necessary purchase personally. I've ridden for over 30 years and have always been fine with skate shoes. Your feet will get bounced off the pedals when you are going through massively rocky or rooty sections but that will be the case with specialised shoes as well. The only way you can actually stop this is by riding clipless instead, that I can definitely recommend as worth spending the money on.

As the other commenter said you want something with a fairly flat sole and squishy rubber the teeth can grip onto and most skate shoes fit into this category just fine anyway.

More recently I have been riding in some trail running shoes which have big knobbles on the bottom which seem like a bad idea. You can't get you feet to lock onto the pins in any position due to those knobs but once you get the pins in the right place on the shoe the pedals hold me well.

Personally I would spend my money elsewhere if you already have shoes the pins will grab on to just fine, I'm not trying to argue specialised shoes won't be better (although I've never tried them personally to know how different they are) I just think if you are gonna go that route clipless are going to be much better cause they literally hold your feet.