We don't have a volume control on the fixed shower head or the handheld in our master bath shower and it is not a problem at all. So in my opinion it is not necessary.

Most pressure balance valves don't come with a volume control, most thermostatic valves do.

To answer your question, no it's not necessary, but I prefer them.

I think it depends on what you're using it for? I have a thermostatic (hansgrohe III) valve that controls my showerhead and handshower, which has no volume control. My husband and I don't mind it, but my kids (now 3 and 5) are terrified of it (it's too strong) which is annoying since the main point of my installing it was for kid rinsing.

It isn't necessary, but nice to have. I also purchased a Hansgrohe Metris
thermostatic valve w volume control and diverter. We wanted our
hand-shower and rain-shower head to work either separately or both be on
at the same time. Also DH likes a very forceful spray and I prefer a
lighter one. If it was any other bathroom besides the master I wouldn't
bother, but it was a must for me.