If segwit ends up having bugs, as some suspect it might, bitcoin cash is the logical replacement as every bitcoin older will own same number of coins. If that happened (segwit disaster) every bitcoin holder will support bitco7n cash, and with larger block size network scaling problem is taken care of.
Bitcoin cash is a segwit insurance policy.
The normal bitcoin chain could still choose to implement the bitcoin cash solution on the main chain.
Imo it is nothing more than an alt with a nice value at this point.
Insurance policies usually don't need activating, but if the event you didn't insure against happens, you can't then insure yourself after the fact.
If the main chain adopts segwit, and it looks likely, and then segwit blows up, which is possible, then having bitcoin cash already up and running will save 'bitcoin'.
Of course, there is good chance it will not be needed, but I'm glad it exists!