to convey happiness

The most obvious and widely thought reason a cat purrs is to convey happiness.

for a lot of reasons

Cats purr for a lot of reasons; and though no one can say for certain what inspires them to create these rhythmic vibrations, there are many theories as to why our feline friends create such endearingly peculiar sounds.

possibly as a way of reassuring themselves

Cats purr when Kittens are afraid or distressed, possibly as a way of reassuring themselves.

as a form of communication as a way to calm themselves down when they're feeling anxious or stressed

Cats purr as a form of communication, but also as a way to calm themselves down when they're feeling anxious or stressed.

For example

This is when a cat purrs during a time of distress to calm itself down and self-sooth, for example during a storm or when they are in pain.

for self-healing

But it seems that purring can also be a sign of stress or injury, leading researchers to suspect that cats purr for self-healing.

to indicate contentment and happiness, perhaps when Kittens ’s curling up with you on the couch or getting a nice behind-the-ear rub

Usually, a cat purrs to indicate contentment and happiness, perhaps when Kittens ’s curling up with you on the couch or getting a nice behind-the-ear rub.

to express happiness and contentment, yes

Cats may purr to express happiness and contentment, yes, but purring is thought to indicate a variety of other emotions as well.

Contentment and happiness

Contentment and happiness are not the only reasons why cats purr.

to express Cats's happiness

Cats will purr to express Cats's happiness, but Cats might also purr in times of pain or distress as a way to calm themselves.

your cat

This deep, rumbling noise signals that your cat is content, which is often why cats purr when they are being petted, or when they’re sleepy.

at a frequency known to promote healing

Cats purr at a frequency known to promote healing, so this behavior may be a self-induced healing or soothing mechanism.

humans

Cats purr as a way of communicating to other felines and humans.

for communication

So, cats purr for communication, and depending how you interpret why ill cats purr, purr is either for self-soothing or maybe healing.