Why is evidence required in order to convict possible criminals?

There's no exact list of things which one may 'require' but there are key pieces of information which can speed up the conviction. 

1) A concrete witness is extremely helpful as it gives secondary view on what occurred. Multiple witnesses can also be helpful as a multitude of answers all of which are the same essentially gives said criminal no way to hide/defend themselves.

2)Evidence, yes this sounds quite obvious but there are different types of evidence. Photo evidence while useful can be tampered with so make sure what you have is legitimate and clear. Do not use photo evidence that does not portray what you are trying to explain poorly. Clarity is key. Video evidence is virtually the best thing to obtain. Security footage is helpful although as I said, clarity is key.