You can plow fields with them if lack the money or resources to own a tractor. Also leather.
Plowing tends to be something you do with male oxen, rather than dairy cows.
They can be used, but it's not common.
And I would assume that for most people who lack resources or money for owning any form of tractor, that purchasing cattle is also expensive for them. Not to say it isn't a solution.
And, given this hypothetical situation where everyone has turned vegan I doubt that many would appreciate the slaughtering of the millions of cows just for leather. Really, that leaves a lot of cow left over, seeing as how cows killed for meat today still have their hides tanned into leather.
And I suppose the only reason to use their meat is in order to feed pets.
Poll