Ambiguous replacement for both your and you're used to mask the user's ignorance of proper usage.
It can be easily passed off as a typo because depending upon context it can be assumed that either an apostrophe was accidentally added or an "e" was forgotten.
The use of this word allows the user to appear cavalier as opposed to ignorant.
You'r disrespecting the English language by so blatantly misusing 'your' when you clearly mean 'you are.'
For when you REALLY don't know when to use "you're" or "your".
Can be turned around to be clever if it is applicable both as a possessive and as the short form of "you are" (example 1); humiliating otherwise (example 2).
Example 1 - when one is in possession of a homosexual whilst also being one themselves.
Kevin: "What did you just say?"
Melvin: "You'r gay."
Example 2 - when you failed Grade 1 English.
Melvin: "That's not my homework, it's YOU'RS"