Carts also seem to be another control tactic. There are only some who are permitted to do cart work whereas other 'publishers' must go D2D to get their time. Not sure what a JW must do to qualify for cart work.
It seems the carts also make it easier to extend the day in ministry. Since JWs can sit and drink coffee, eat and talk to each other close to where the cart is set up, there then is no need to take a break elsewhere such as a coffee shop or restaurant.
I've observed JWs with carts outside ice arenas and other public venues, late in the afternoon, beyond the time when publishers would usually be out knocking on doors. What I found odd though, just as increasing numbers of parents were arriving with their children to skate or play hockey, the JWs would quickly pack up and depart.