For this question a lot of people was seeking for the answer, however we still do not know the exact answer to the „Why” and neither the „How”.
The first one seems more easy, as we can discover easily in our own cat. Mainly they are issuing this voice in a comfortable situation, int he lap of the owner, waiting for dinner, during eating tasteful snacks.
Therefore the scientist thinks that they are announcing with purring their statisfacion and comfort feeling.

However they are purring in incomfortable situation also, for example at the doctor or the female cat during droping. Most probably the reason of it is that the frequency of the purring has a calming, even curing impact on the bones and on the organs- the positive impact of this vibration has been discoveres and used in the human healing also- of course not by cats, but by suitable equipment.
Purring in an uncomfortable, even painful situation can be a self-curing therefore and this might be the reason that for the cats is is very rare to have bone-or muscle abnormality during growth and even if such an injury occur it heals very quickly.

The question of the how is much more difficult, there are several theorys exist but all of them have failed already. The most common theory was that the voice is created by the rhythmic pulse of the Larynx, but since cats who had larynx intersection due to medical reason is able to purr also, this theory cannot be true either.

One thing is sure: Only the felines with bony language bone are able to purr, like jaguar, puma or cheetah, and surprisingly not only the felines, but for example the rabbit or the racoon are able to create similar voice as the sign of theis statisfaction. But the lion cannot purr, with its cartilaginous, flexible language boneit is able only to howl.

Many times the purring is accompanied with kneading (the „treading” in the lap of the owner, possibly with the release of the nails), which is a habbit can be linked to the kitten time, as the little kitten during sucking kneading and purring in the belly of her mum to stimulate the milk production.
According to few theories by kneading int he lap of the owner the cat life trough this comfortable feeling again, other theories say the kneading means a kind of way looking for security and a balanced cat does not have a need for it.
However this is well-known, that half closed, slowly blinking cat eyesmeans statisfaction for sure, therefore if these three things are come together, we do not have a reason to worry.

In case of a possible sickness do not be necessarily calm if our cat starts purring, since this can mean a type of pain also as highligted above.
It can be a cure for our pain as well, what our cat can also feel- with a daring consequence we can say that he cat owners can be even more healthy and balanced thanks to their velvet- feet favourites.

Zsuzska