They want you there to answer questions.
Workplaces value availability because it's too difficult to design coherent systems and to document them. Instead of that boring old stuff, in the modern workplace, we go fast and break things (or never get them fully working in the first place).
Then we expect workers to be always around so they can be reached on slack when there is a problem.
But hey, I like surfing, and you can't 'reach' a person while they are surfing. ¯\_(ツ)_/¯
I don't like it either, and I feel like I could contribute a lot more effectively on the fixed price items if I wasn't constantly scrounging up T&M work, maybe I'd even have time to think up decent solutions instead of justifying why I have to spend another 2 hours on issue #blah, but it's an example of why required hours exist.