It is easier for the average film unit to have a less than perfect physical match for their actor than to have someone on set who does not fit in, who grates. Inexperience, chattiness, absence from set. To the untutored eye, shooting a film looks a lot like a bunch of people standing around doing nothing. The sensitive may note that even in doing nothing there is an atmosphere of high tension amongst the crew. This is because people are focused, alert, watching, expectant.
Nobody wants to be the one who slows down the team.
A stand-in who is on his cell phone or makes too frequent trips to the honeywagon takes more work than a production team needs to expend. That's eight working hours. Absolutely it is. The observant actor can learn about acting, camera, lighting, timing, etc and often from masters of the crafts. Not all stand-ins are actors of course.