With the mpm-itk that is a multi-processing module (MPM) you can run each virtualhost with it’s own user id and group id. This is basically used to make a shared hosting secured, since you don’t have to make all virtualhosts folders readeable by apache user.
mpm-itk is based on the traditional prefork MPM, which means it’s non-threaded; in short, this means you can run non-thread-aware code (like many PHP extensions) without problems.