Good question, sorry for late reply!
@ol => Ordered List
@ul => Unordered List
I use them at the project level (project = list of tasks). It helps me remember if the tasks listed under a project are to be completed in order or not so that I don't have to ask myself that question later.
I don't use them to "filter" by contexts. I use them as visual a "helper" contexts during review or when I take actions. It may not be GTD compliant but it really helps me, especially when I have lots of projects to go through and/or review.
The most valuable argument to use these contexts is that... It works for me.
I opened up an old list file. Here are 2 examples where I used
@ul at the project level: