My partner and I wonder about that all the time! Why all the time and expensive and effort to take an infant or very small toddler to Disney? I don't "get" it, obviously. The child is too young to really enjoy most of the activities, they aren't going to really understand what's going on, and they aren't going to remember much of it, if at all. I have a very good memory (nearly photographic) and I only have a few memories here and there of things when I was a toddler -- and some of those memories are probably influenced by family stories I was told, too.
But I have to admit I often wonder these days why such small children are being taken into restaurants (often late at night and I mean high end restaurants), bars, museums, movies, etc. All places I personally think are inappropriate for children so young.