Minecraft wiki entry for Best_biomes_for_homes, associated text: Flat landscape for easy building and traveling.
Plentiful tall grass for initial seed farms.
The presence of animals offers much-needed meat, and resources like leather and wool. Compared to other "green biomes" like the forest, the spawn rate of passive mobs is not increased, but the lack of obstruction makes them easier to find and offers more spawning areas that are usually covered with trees.
Villages that offer great trades, potential chest loot, and maybe a good shelter.
Plains villages are also the only villages that can generate large houses, which have 4 beds.
Horses and Donkeys spawn exclusively here or in the savanna. Horses offer great mobility and combat advantage, while donkeys can be used for extra storage. In Bedrock Edition, this is the only biome where donkeys can spawn.
The sunflower plains is the only place to find sunflowers.
The abundant tall grass can sometimes hide you from monsters (or surprise them), even when you're on the move.
Both water and lava lakes spawn here, often easily identified due to the flat landscape, and can provide plentiful resources for various advanced tasks.
Fewer trees mean the undead have little protection from sunlight.
Cornflowers generate here, used for blue dye.
Bees and bee nests can spawn. This allows for honey farming which brings farming of honey bottles, honeycomb, honey blocks and beehives.
If you are on a multiplayer server and at your base, the lack of cover makes it hard for an enemy to sneak up to your base.
Plains villages have large amounts of hay bales, which can be made into bread.
If you are looking for ores, caves entrances (sometimes ravines) can be easily spotted from the surface.