You can place spawners where you want, on your lot, on community lots, almost anywhere. It's essentially if collectibles never appear in your world. In custom worlds, for example, or if you really can't manage to find a particular collectible you want. For example, in my world I waited years and years to find a pink diamond but it never appeared where it should normally appear, so I eventually got enough and placed a rare gem spawner on my lot and the day after, I found a pink diamond there. I also like to place butterfly and wild flower spawners on my lots just because it makes the lot look prettier.