Definitely seems like a lot of guys keep their feet in the Reserves at least until they're well and truly established in the majors, furlough-proof lifeboat among other reasons. My last CO (O-6) was also a fairly senior captain with one of the big boys (UAL, I think?) and was fond of saying at drill weekends, "Doing that job allows me to do this job." For a long while the VAW SAU was a FedEx flying club.
It is, without doubt, a huge benefit when it comes to junior life at an airline. You'll still put food on the table if the music stops. There is probably some scenario where the music stops in both places (COVID actually came close, since it affected most of our airline folks equally, and there are only so many long orders that can be funded), but it's a lot better than nothing.