Mining at a fixed difficulty

Hi everyone,
I'm quite new to cryptocurrency mining and I recently read that for example when mining Myriad-Groestl, it can be worth setting a fixed difficulty based on the average difficulty observed with an hour of mining.

1. What is the effect of setting a fixed difficulty, isn't it supposed to be optimized already ?
2. Is there a parameter for fixed difficulty in ccminer ? I've been reading through the parameters section of the readme and couldn't find it, maybe there's something a little more obvious that I wasn't able to understand myself

Thanks for the help guys, have a nice day