What does the process Epchost.exe do?

Epchost.exe is using a lot of memory on the Client PCs.

When entering data in the Contributor Web Grid, if Task Manager is opened, you can see the Epchost.exe process using a lot of memory, but what is it doing?

Epchost.exe is actually the 8.2 (and upwards) Planning client component process. It caches information and offline functionality that was performed by the epOfflineGrid.exe and dllhost.exe in previous versions (8.1 and before). Basically it holds information in memory as a live cache - this is completely by design. It takes a snapshot of the workflow state (elist, users, workflow states) for each application and we hold it in memory underneath this process until the application state changes (GTP etc).

This is completely by design. We expect to see this using memory (and significant amounts) even when the application is not being used.