For most of us that go through 28 iron sets a year, we do fitting by default.
Back in the stone ages, before the pursuit of ho took over my life, I went to Dana Upshaw for a fitting. My gamer 5 iron went 171. Dana's fitting, which changed clublength, lie, shaft and head, made a different 5 iron with same loft go 192.
So a fitting can be revolutionary - it depends on your starting point.
And it is Dana's fault for making me look for 17 yards more from a new driver every year.