Here is a clip from an article explaining all the types of compression, it is a pretty in depth article, and goes more towards studio type compression then pedal compression, but still gets the points across.
What are they used on?
An OPTO compressor is good at taking care of macro-dynamics. If you have a song that starts quiet and ends loud a VCA compressor will do nothing at the beginning and will smash the signal at the end (depending on your settings of course). An OPTO compressor will smooth things out and will have a more consistent level throughout the song. When you want to tighten up your mix a little bit without losing transients, use an OPTO compressor on your mixbus , but be careful of pumping which will boost the low end content. To prevent this from happening you can use a high-pass filter in side-chain mode. It also works really well on bass , guitars , and vocals . But then again it depends on the performance, if the performance has lots of transients you might be better of using a different type of compression or perhaps a combination of two.
If interested you can put into a search field on any search engine “Optical compression vs FET vs regular”
and there are all sorts of websites with good in depth write ups about this subject.