SSL cert in the F5 and in the respective web server

Even if you are re-encrypting to the backend servers, you can reduce the SSL key sizes on the backend, compress and cache data on the BIG-IP, and use one connect, all reducing utilization on the backend servers.

You can also then utilize cookie Persistence, perform URI switching, as well as implement additional security services.

In general, even if you need to encrypt to the backend servers there are numerous benefits to placing certificates on the BIG-IP.