People with money enjoy having a place to get away to. If they live in the city they often want a weekend place at the beach, the lake, or the mountains. Depending on their finances they may not need to rent it out, which allows them to use it anytime they choose and also leave their personal things in it and not worry about them.
Vacation homes are often in special locations that can appreciate more than average, because of the limited availability of a feature such as being on the water or having a 50-mile view. A lake or beach house probably won't appreciate as well as index funds, but they are much better investments than boats and RVs. They are also great for making memories and passing down over generations.
As rentals their demand is usually seasonal, which can work for or against you depending on your needs. If you like the beach, but like to avoid the vacation crowds, renting it out during the peak season works perfectly for maximizing income. If you by a ski cabin, but don't like the cold Winters, rent it to the skiers, then you can enjoy the other seasons.