I don't have a lot of people over for meals right now, though I hope to change that. But when I do, or when I'm arranging to go out for a meal, I always ask "are there any dietary restrictions?" Otherwise, it's all too easy to inadvertently leave someone with little or nothing that they can eat: for example, someone I know has to avoid dairy altogether. Knowing that, it's easy to work around. If I didn't know, I might serve, say, cucumber salad with a sour cream dressing, a curry with a few tablespoons of milk or cream in the sauce, and ice cream for dessert, leaving that person with nothing to eat but plain boiled rice.
Knowing, even if I didn't change the main dish, the salad could have a different dressing, and I could be sure to get sorbet as well as ice cream.