The BIG-IP has two caching engines. One is the RamCache or FastCache. This is using memory to cache objects. The max memory that would be available for this is 3GB so it is very good for hot objects most likely in a reverse proxy scenario.
The BIG-IP Web Accelerator product and module for LTM support caching. The module running on LTM (6400 model and above) uses around 36GB of hard disk for caching. This supports static and dynamic caching. The BIG-IP Web Acccelerator stand-alone uses about 120GB of space for caching. The main reason that BIG-IP uses caching in any of these products is to speed up the delivery of content to users and offload the backend web servers. In most cases, they are used as reverse proxy caches, but the BIG-IP web accelerator can be used as a forward proxy as well.