As a live site that stores data, for example live chats, if you are away for a while and not active, you are still having this data stored, it can slow down your phone, laptop etc, it also means you will have problems logging in and off because it thinks you are still here. So if you are not active for 30 minutes the system automatically logs you out. You can just log back in and continue where you was. Inactive means no clicking anywhere for 30 mins.