In Star Wars: A New Hope Owen talks about how he needs Luke for the next harvest and stuff, but it's not like they're tilling crops or anything. Don't machines just suck water out of the atmosphere, then you just haul in barrels into town?
They have lots of land to cover, and vaporators will occasionally break down. It isn't just "go get water, take water to town". Yes it is easier with a speeder, but it's still a lot of work to get everything quickly so that the vaporators are ready for the next harvest.
There's also work of maintaining droids, machinery around the farm (like the skyhopper and the speeder) that take up tons of time. Also having to patrol the territory for raiders, jawas, thieves, that sort of thing. While they do mention having farmhands to help out, it clearly wasn't enough for all that needed to be done.
The majority of the work would likely be maintenance on the vaporators and other equipment (such as droids). Tatooine is a desert world. That means sand. Sand gets into all sorts of gaps and crevasses, which will interfere with the working of the equipment. If the vaporators aren't working at full efficiency (or at least close), they won't pull as much water out of the air, meaning a smaller harvest or maybe even no harvest at all.
They would also have to monitor the vaporators to ensure that no one else (other farmers, scoundrels, or sand-people) raids the vaprorators' storage containers.
The assumption that a moisture farm would involve little work, because you just have to haul the water into town, is like assuming that a modern farm involves just a little work because you just have to plant and then haul the crop into town. While planting season and harvest season are the two busiest times of year for farmers, they don't exactly sit idle for the rest of the year.