I wondered the same thing myself , when my online master first gave me safe words. I thought it was a tad silly more than anything else.
I thought this is online, how 'real' can it get....and wondered if it was possible to get myself into a position that my limits would be tested.

I wanted my submissive needs met. I wasnt going to get that from a play only situation , I want my limits tested and how was I or anyone else going to do that if I didnt allow my self to get in a position where they could be.

I have read it here in several forums.....and learned very quickly

Masters do know best.

Listen to what they tell you, always

The only exception I can think to that would be if your Dom was very new and not experienced.