A slab is essentially a dedicated portion of a full server, and as such, can do nearly anything that a full server is capable of doing.

A single slab is capable of running at least 10 typical shared host web sites. So a good use for a slab would be a web developer that wants to host it's own customers. The web developer would be able to install whatever software was needed for the hosted sites, and install and maintain the sites in an environment that isn't at the mercy of other developers. The web developer would be able to resell shared host ability, without having to purchase the infrastructure to provide the service. If the needs of the developer expanded beyond the capabilities of a single slab, multiple slabs can be joined together to increase those capabilities.

A slab can run nearly any server software that runs on linux. This means that if you have a "legacy" web app that needs an older version of php, for example, you could install it on a slab, so that the app could run. Instead of trying to find a shared hosting provider which supports your needs, you can install the software you need on a slab.