Simply put - it reduces file size and load time for websites without any PHP code changes.
Here is come example PHP5 code: $minified_html = PHPWeeMinify::html($any_html);
The PHPWee HTML compression technology is totally new. It actually parses HTML documents and removes unneeded spaces and verbosity without changing meaning in any way. The output is generally at least as W3C compliant as the input.
If you care to take a look at Google Page Speed tools - page loading speed is becoming not only an important part of web user interface design - but is also starting to affect organic search positioning. Poorly optimized code may be slowing the web down by as much as 20% - even after GZip compression is applied at the server side.
We typically see a 15%+ improvement on page load speed on un-optimized websites by applying phpwee. Even Wikipedia and Github.com can get a boost. The benchmark tests can be found at searchturbine.com/php/phpwee.