A lot of people do have goats just to keep the grass down in a field. You obviously do not want a field ornament, it must be useful as you say. So, the obvious choice is milk or meat. If for meat, then castrated billies would be the choice. If you want them for milking, then you need to make sure you are shown by an experienced goat keeper how to milk. I am sure you realise that milking needs to be done properly to prevent the goat being uncomfortable, or getting the dreaded mastitis.
As to the usefulness - well if you intend to eat the meat yourself, one of the bigger breed of goats will give you a fair amount of meat. The female milker, is expensive to buy, then there is the stud fee to the billy, plus all the feeding costs of keeping a nanny goat. You really need two goats at least, as they are herd animals and prefer company. If you want the milk, the kids need removing from their mum and bottle feeding, which is time consuming, and you need to decide what to do with the kids ie. selling them on. Nanny kids will be no problem, but the odds are you will end up with a male kid. They are harder to rehome, unless they are well bred.
You also have to think about things like who would milk the goat if you were ill, on holiday etc. Other people can look after hens, dogs, cats etc, but you cannot expect a next door neighbour to come and milk a goat!!
There is the cost too, not just buying the goats, but feeding - they can eat a massive amount of hay during the winter months. Then there is worming, innoculations, CAE tests etc. to be taken into account.
If possible find a local breeder, and ask if you can visit to have a chat, look at their goats, and ask as many questions as you want. Most people would be happy to help in this way. Better you know exactly whats involved before getting the goats than after, and then finding it was not for you.