Partly to make best use of them, but also because the soil tends to be richer and can support more plants.
I now have raised beds on my allotment mostly to save all the digging but also you can plant closer together. It makes life much easier as the earth is not getting compacted by treading on it. Purple sprouting broccoli has done really well and the artichokes have gone mad.
Not everything can be planted much closer together, particular those vegs which produce their harvest to eat above ground, vegs like cabbage needs lots of room, even harveted as baby veg their top growth is boarding on huge. Brussels take up a lot of space above ground and need space, sweet corn can be planted closer together but can grow up to 6ft tall or higher, needs along growing season and in order for the cobs to ripen needs to see the sun and feel it's rays.
Fruit, like currants and gooseberries need air circulating around their middle so don't do well planted close together.
Lots of plants can be planted closer though, a raised bed which has been given a good dose of well rotted fertiliser, coupled with regular hoeing also seems to produce less weeds the following year.