The desert lake is a biome that can be found in the Overworld and it is an area that is made up of sand, cacti, and dead bushes with numerous rivers flowing through and around it. You can also find sugarcane growing alongside rivers. It is a part of the desert biome and can be classified as a part of it rather than a variant.
Desert lake biomes are flat and barren just like a regular desert and thus their vegetation consists of cacti, dead bushes, and sugarcane. They have little to no raid making the weather quite dry. The surface is completely made up of sand blocks which are supported by sandstone blocks that generate underneath. If you dig deep enough you will find other blocks such as stone, gold ore, coal ore, etc.
The only non-hostile mobs that spawn in desert lakes are rabbits and squids since there are rivers flowing. Villagers, cows, cats, and sheep can spawn in villages only. Hostile mobs that spawn here include spiders, skeletons, creepers, slimes, husks, zombies, endermen, witches, zombie villagers, spiders, and zombie jockeys.
The desert lakes biome is a part of desert biomes which are barren biome with very little wildlife and vegetation, cacti, sugarcane, and dead bushes are the only vegetation that is generated in this biome. Players can find structures such as desert temples quite commonly in desert biomes.
A. Yes, desert lakes are hard to come by making them somewhat rare.
A. Desert lakes are a relatively rare biome that is found in desert biomes. They serve no real purpose other than decoration.