I have a perfectly good motorcycle helmet (one that encompasses the whole head, including the chin) that I never use (I bought it for lessons, but never bought a motorcycle). However, I do not have a skiing helmet and have to borrow one each time which costs money and is not as hygienic as I'm really comfortable with.
So I wondered why I (on the level of a skiing beginner!) couldn't just use the one I own. However I remember reading on some SE site (possibly here, I don't remember and cannot find the post) that not all safety helmets can be worn interchangeably. For example, a bicycle helmet shouldn't be worn for climbing — but I don't think falling down on snow (repeatedly) would be the same as being hit by falling rocks. Also a motorcycle helmet is significantly different from a bicycle helmet.