It is to scam you out of your money. They offer prices significantly above market, around 3-5X times what you would pay for a taxi in the area. Some of them will offer more elaborate plans like taking you to visit certain places and they will but again will charge you significantly more than a typical tour bus.
Actually, I was there a month ago and this happened to me all the time. They would call for attention or sometimes strike a conversation first, usually starting with Where are you from? or Where are you going?. I could also clearly tell that I was targeted more than others by a significant margin since I do look like a tourist, this increased to another level when I had my camera gear out and some would get right in front of where I was shooting.
One of the city guides given at the bus station even said about taking motorbikes:
Motorbike Taxi: If you like having helmet hair, a filthy place and an
empty wallet, go right ahead.
One in Hoi An, a few days before reaching Saigon, even convinced me to take one as he said I was going to a car-free part of the. That turned out to be false and the charge was about 4X times what it cost me to do the way back using a metered cab. This scam is prevalent in all parts of Vietnam.