This used to be a popular local spam technique and back in the old 7 pack days MyMaps were a ranking factor and could give a boost. I used to build one or 2 for clients in a couple different trick ways, just to strengthen trust in their location - sort of like a citation. Nothing spammy, just one or 2 and it seemed to help - for awhile.
I witnessed some Chiropractors a couple years ago that would try to leapfrog each other in the SERPS in part by having cheap overseas labor build a bunch of MyMaps. At one point one of the Chiros had 1,000 MyMaps.
But I highly doubt MyMaps are a ranking factor any more and have had discussions with leading pros who think the same. Likely lost favor with Google as a ranking factor due to all the abuse. It was just too easy to game.
I think people are just selling a bogus out-dated service, SAYING it helps even though it does not any more, to folks that don't know any better.