I would assume it would be so viewers could tell the difference.
Nova and Intrepid class starships were designed and built around the same time (early 2370s), so they probably used the same kind of transporter.
Defiant and Danube class vessels were designed a few years earlier. The Defiant class was actually designed to combat the Borg.
Sovereign class vessels don't use the same transporter effect as Galaxy class, on Sovereign class vessels, transport is a bit faster, with vertical streaks and a wavy motion.