What's the use of abstract syntax trees?

They represent the logic/syntax of the code, which is naturally a tree rather than a list of lines, without getting bogged down in concrete syntax issues such as where you place your asterisk.

The logic can then be manipulated in a manner more consistent and convenient from the backend's POV, which can be (and is, for everything but Lisps ;) very different from how we write the concrete syntax.

The main benefit os using an AST is that you separate the parsing and validation logic from the implementation piece. Interpreters implemented as ASTs really are easier to understand and maintain. If you have a problem parsing some strange syntax you look at the AST parser , if a pices of code is not producing the expected results than you look at the code that interprets the AST.

The other great advantage is when you syntax requires "lookahead" e.g. if your syntax allows a subroutine to be used before it is defined it is trivial to validate the existence of a subroutine when you are using an AST - its much more difficult with an "on the fly" parser.