"On my way" tells the other person nothing useful. They still don't know when to expect you, whether to start without you, or how worried to be. It also quietly signals that you know you're late but hope vague reassurance will be enough
"I'll be there at 7:20" does several things at once. It shows you've actually thought about it. It gives them something to plan around. And if you're more than a few minutes off from that estimate, you have something specific to update rather than just going silent.
The specific time also tends to make you more accurate because saying it out loud forces you to actually calculate instead of just guessing "soon." I started doing this a few years ago and the number of times I've walked into a situation where someone was visibly annoyed dropped pretty noticeably. Small thing but it costs nothing.