Guitars, like other string instruments, have multiple ways to play each note. Depending on the note progression, hand position needs to slide up and down the neck of the guitar to ensure the necessary groups of notes are all easily reachable from the current position. Tablature solves this issue by telling guitar players where to place their hands. Orchestral sheet music involving stringed instrument parts often include both the standard staff and tablature.
Of course, the simplicity of tabs also make it a great way for the community to spread knowledge. A simple tab can be written down in notepad in minutes and passed along freely, whereas sheet music requires specialized software and hours of time.
This makes it very attractive to beginners who aren't looking to become professional musicians, but simply want to mimic what they hear on the radio for their friends. I can either sit you down at a piano for two years and teach you sheet music, or give you a few weeks and tabs. Will you be playing perfectly? Of course not - but you'll be enjoying yourself.
Mind you, piano notes are already in a 1:1 correspondence with the staff, so hand placement is greatly simplified and tablature would be redundant. When the bar to musical literacy is to learn how to read a notation optimized for piano, it becomes a pointless question to ask why piano players achieve literacy.