* The consultant coming to a project for a short period of time
* The software architect, observing the code layout periodically (for example to identify where lower layers of the software depend on higher layers)
(And then, there are projects you look at very briefly. Just a few days ago, I used Sourcetrail to explore a certain research CLI application. I mostly needed to know how it does a few particular things, and Sourcetrail was useful for finding my way around the exceedingly complex class graph quickly.)
There's no good tool out there for reading source code. Sourcetrail is a step forward that's useful for anyone.