Description (from Recipaedia)
Tall grass randomly grows on well lit and humid grass blocks. You can obtain grass seeds from it, which can be used to plant more grass. Can be used as a decorative plant.
Food
Horses, Camels, Donkeys, Cows, Bulls, Gnus, Rhinos, Reindeer, Zebras, and Bison will eat tall grass.
Animal Herding
Any herbivore will go after tall grass or flowers when dropped. Use this as luring out tactic to hunt and gather animals for domestic use.
Sneaking
Because there are so many dangerous animals lurking around, you may need a place to hide from time to time. When you go into sneak mode, you are able to hide behind anything with a minimum of one block tall. This includes tall grass blocks. They are also great to hide behind while shooting birds. They will not be able to see you, so they are less likely to get scared and fly off.
Trapping
Line your spike pit traps with these to further trick the creatures that wander by. (Confirmed by SteppenwolfMLP, 26/06/2022).
You can also throw tall grass on top of your grass traps to lure cows, bulls, and gnus right into your trap.