From Federico Cargnelutti's blog post:
Features include file transformations (e.g. token
replacement, XSLT transformation,
Smarty template transformations), file
system operations, interactive build
support, SQL execution, CVS
operations, tools for creating PEAR
packages, and much more.
Of course you could write custom scripts for all of the above. However, using a specialized build tool like Phing gives you a number of benefits. You'll be using a proven framework so instead of having to worry about setting up "infrastructure" you can focus on the code you need to write. Using Phing will also make it easier for when new members join your team, they'll be able to understand what is going on if they've used Phing (or Ant, which is what Phing is based on) before.