basically saying if the exchange picks the wrong fork for you you can lose everything, and you need to get things into your own personal wallet so that if/when it splits I would have bitcoin on both forks or something just seems super involved and I would expect a decent chunk of other casual type's like myself to feel similarly.
The idea behind getting your xbt out of the exchanges and onto your own soft/hard wallet was also that some of the exchanges are not supporting bitcoin cash at all.
This roughly translates into : you get robbed out of something you would have owned had you been in control of the private keys.
How it works (well "worked" since this fork is now done, but this remains valid for other future forks, on whatever cryptocurrency) :
say pre fork you own 1 xbt
*fork happens*
you now own 1 xbt + 1bch (BitCoin Cash) on the "new" blockchain (because it has already be mined up to this point, they are not resetting the whole thing, but from this point on and forward, the rest of the chains will be different)
So essentially, exchanges saying "we are not supporting btc" are not giving you the 1bch you would normally rightfully own.
Note that you have not by any means doubled your money. Bitcoin Cash's value will probably be much lower than xbt. Also xbt's value might go down for whatever random reason, and your 1xbt + 1bch's total value translated in fiat might be lower that what you could have gotten had you cashed out your 1xbt pre-fork. Or it's value might have tripled, who knows !
The important thing here is, as Arative said : control your keys (not only for forks, but also because the exchange itself might get hacked, it has happened before several times)