"If I own Bitcoin, do I automatically own Bitcoin Cash too?" - as well as I know it depends on where you stored your btc before the first of August. If it was on the exchanger or some online wallet, you probably won't get your bcc.
"Why was a fork necessary to create Bitcoin Cash?" - I think it wasn't. Something had to be done and with btc is was done indeed. Bitcoin has a segregated witness technology now, so it has solved the problems with transactions. Bitcoin cash was supposed to be a back up plan but, as too many people got interested, it was released anyway.
I thought this time is BCC is just a backup of BTC since BCC has some modifications to strengthen BTCs weaknesses. Just an observation but maybe we're right or wrong but in this case I also thought BCC developers know what they are doing or they're just trying to challenge BTC's popularity or rival other low end alt coins to keep BTC's position as Top Virtual Currency in the world.