When you're fretting an acoustic guitar, there's an added complication: The fingerboard extension (that bit over the body).
Mostly, this part of the fingerboard is glued to the guitar top and usually has very little support underneath. You can't just start hammering frets in here without running the risk of badly damaging the guitar top or the bracing beneath.
There are ways of pressing frets into this section but they're slow and a little clumsy. What I (and most repairers) do is to use a 'ballast' of some sort to absorb some of the hammer-blows' energy. This means using one hand to hold a relatively massive (as in mass rather than size) object underneath the fingerboard extension as you hammer frets home with the other. This works but is also a bit clumsy and a bit awkward.
Years ago, Taylor Guitars went a long way towards solving the problem. They used a cast-iron device they called a Fret Buck. The buck sat on the guitar's face and an internal 'foot' clamped up underneath the fingerboard extension to support it. Hurrah for Bob Taylor.
For a while, you could even buy a fret buck. I know because I kept saying to myself, "I must get one of those. I'll put one in my next order." I didn't though. I kept forgetting or putting it off.
And then they disappeared.
No longer available. Anywhere. About six or seven years ago, they vanished. I continued with my contortions, wishing I had a third hand, while fretting. No biggie.
Then Stew Mac worked some magic and began offering their own version a few months ago. Hurrah for Stew Mac. The Fret Buck's not an absolutely essential tool but it does make an awkward and potentially damaging job a lot easier and safer.
And quieter. Hammering frets over an acoustic body is a noisy job and the buck soaks up some of that noise too.
I'm quite fond of my fret buck. Should have got one years ago. ;-)