It's useful if you're trying to maintain X utility at the lowest cost for any X. It's also useful if you're trying to maximize utility at a given cost Y. If you only have a finite amount of money, it's useful.

One might be deciding to increase the amount of utility they're adding, and deciding between vegetarianism and donating more money to effective charities than they were already giving. The limit is how hard it is for the altruist to do each of those things, so if they find giving the amount to achieve an equivalent amount of good to going vegan is less painful than for them to go vegan, they should do that. Caring the amount a utilitarian "should" is enough to grind most people to dust (I, personally, faced an opposite issue of looking at the problem and immediately giving up on morality and becoming functionally an egoist).