As a developer I’m constantly going in and out of play solo to iterate on changes. If my game uses datastores I either have to hack in some play solo special case to not save data, or I have to deal with 6+ extra seconds whenever I close play solo. In tests that only take 1-2 seconds to run, 6 seconds of waiting for this timer is an enormous amount of time to be doing nothing.
This is one of the reasons in-window play solo is inferior to out-of-window play solo.