The key term is “for their benefit”. It’s such a vague phrase that pretty much anything can be turned into a legitimate reason.
In this particular case, it would be really simple to argue that your husband wants to go but simply can not due to health and disability, therefore you need to go to represent him and your family. You can also bring him news from family, friends and all that other gossip that will lift his mood as well as bring back cake and the well wishes of those you see there. All of which will benefit him greatly.
For a simple, single journey, there’s no issue in using the car and claiming it’s to his benefit. If it was regular long trips, it might prove harder to justify.