Generally on camera monitors are for video and film. the monitor is
like a small TV and should show the shooter exactly what he going to
have in finished form. I have a 2 one large and one smaller.
If a studio photogapher was shooting products or a client shot
he might have a tethered laptop or monitor for all to approve.
It also works very well for shooter using mirrorless and dslr"s
that have the connection to mostly shoot video.
I used to produce corporate videos with a crew that
travels. We had a lot of gear and always had a 12"
confidence monitor where everyone could see the shot.
The viewfinders or LED's on most cameras are very
useful but a larger visual really helps.