As an example, they do all of the scut work at the hospitalist program near here, basically functioning as interns for life. They do all the annoying discharge and admit stuff, while the patient gets to do a full eval of the patient. Less complicated things get bumped off to them so the physician can focus on the more complicated patients. The PA basically does a lot of the BS paperwork and administration, freeing up the doctors to do a lot more doctoring and a lot less annoying regulatory crap. Each doctor has a PA, which basically lets them nearly double their productivity, all for a fraction of the cost of two physicians (hospitalists are paid on the low end for work here because we're in the NE and there's more supply than demand in the better hospitals, pay's only about 80k to start, 100k with experience, while the hospitalists start at 190 and can make up to 240).
In other environments, such as derm, you can oversee four PAs that do procedures for you, while you just do the evals and tell them which procedures to do- you diagnose, they cut. PAs will do all of the uncomplicated and low-paying follow-up work, as well as the high-paying and fast procedures, allowing you to substantially increase your earning power.
In primary care, they can see your uncomplicated patients, saving you time for the more complicated ones. I read a report that put the average amount of money earned per midlevel at about 40k over expenses, so if you were overseeing four of them, you could pull an extra 160k per year if you were willing to stomach the liability. Far more if you added ancillary services like cosmetic botox injections, laser hair removal, and other crap.
In psych, psychiatrists will often manage the more complicated patients and new consults, while having their PAs run through all of the med checks that come in ever 3-6 months, which essentially just amount to a quick "you doin' okay? Here's your script" and a lot of paperwork. You can make a fortune off of midlevels in psych if you know how to run things efficiently.
Now, the trouble comes with when you teach a midlevel too much and they start to think they can work without you. Let them do some of your job, but don't make yourself replaceable if you want to avoid going the way of anesthesia.