I'm not sure what their business model is. To be seen, but not to be actually doing anything of substance?
When the trolley* work first began, the original training was to hold up magazines and try to engage people. Rather like the old Witnesses used to do outside railway stations and the like. This led to a huge number of placements per month. We used to get a lot of people coming up to talk too.
I believe this was happening in a lot of other places too, and even photos in the old Kingdom Ministry for meetings at the time used to show Witnesses standing holding mags, as the "model" of how to do it.
After a few months though, reports must have got back to "Mother" in the USA that this was costing the Org a fortune, because suddenly the edict came down that we should NOT hold out publications, but instead leave them on the trolleys and "direct interested ones" to there for literature.
One of the excuses given was that we are not supposed to be like free newspaper distributors who dish out their copies only to see most left littering the streets or trains/buses. We are supposed to "value" our literature more highly (but surely getting the message into the hands of people is most important? You know, that "giving a witness" thing?)
I realised straight away that would fail, and I also (correctly) worked out that that would soon make the Witnesses staffing the trolleys disengage. If you're not holding literature and trying to catch people's eyes, what is there to do for an hour or more? The answer soon came: chatting among themselves and playing on phones or holding cups of coffee.
Over time, this "direction" became even more watered down: the Org has narrowed the number of items they suggest are made available on the trolleys, and all instruction generally steers people towards the website instead. More recent pictures have shown Witnesses (including GB members) standing at the trolleys using only a tablet.
So the initial success of the trolley work - placing literature and catching people's attention - was sabotaged by the Org itself, once it realised it came at a financial cost. This shows they were not really interested in spreading the message widely, getting more conversations and possibly even increasing members - not if it came with a price tag!
* = In the UK we generally don't call those mobile stands for literature "carts" but "trolleys". A small, hand-propelled container for carrying objects is usually called a trolley not a cart (shopping trolley, dessert trolley, etc). "Cart" is usually used to refer to a larger containerised transport, pulled by one or more people, or by animals.