I guess it depends on how deeply you want to dig into the place of Ravenna in history as the western edge of Roman empire after the fall of Rome itself. I think Rick does a decent short overview of the place of Ravenna, the source of the mosaics and their place in Christian history. If you want more information and historical context then that then a good guide can add that and Ravenna has a lot of history.
The actual logistics of touring the various sites and admiring them is easy enough without a guide. We were there earlier this year and Ravenna understands why people are there and facilitate the touring of the sites with blanket tickets and maps of the various sites.
I would also recommend this under advertised site for Roman style floor mosaics in Ravenna if you have time.
https://www.turismo.ra.it/en/culture-and-history/archaeology/domus-stone-carpets/
Enjoy Ravenna,
=Tod