Initially, Bitcoin Cash were not planning on changing anything other than implementing "Big Blocks"... as they were trying to take over from Bitcoin. When the Bitcoin Cash dev's realised that their fork was going to fail due to lack of support, they implemented replay protection shortly before the fork occurred. This replay protection effectively made them an "Altcoin" and made the two chains completely separate.
Had they not done this, people could have taken a Bitcoin Cash transaction... and broadcast it on the Bitcoin network (and vice versa)... thereby "stealing" coins on the other Network. The scenario being:
- User A sets up a deal to buy 100 BCH from User B for a nominal sum
- User B broadcasts Transaction X, sending 100 BCH to User A
- User A then copies Transaction X and broadcasts it on the BTC network
- User A then receives 100 BCH on BCH Network AND 100 BTC on BTC Network (even though User B only wanted to send 100 BCH)... effectively "stealing" 100 BTC from User B