Answer by Rory Young:
himself used the lion's mane as an example of evolution. He explained that it developed as a means of protection during fighting. He was quite correct that lions fight much more than other cats.
It is unlikely to be coincidence that the only cat with a mane is also the only cat that fights regularly and aggressively and also the only cat that lives in social groups. Whilst other cats will spend solitary lives in their own territories, only coming into contact with a few neighbors, lions actively socialize and in the case of males, as said, fight for position.
Recently, there have been claims that the mane may be more a means of displaying strength. However,the fact remains that whilst other cats and female lions are often killed by bites to the neck, this rarely occurs among male lions.
If a lion is killed by a bite during fighting it is nearly always a result of a bite through the back severing the spinal cord.
It is most likely a combination of protection and a means of displaying health and dominance to rival males and the females of the pride (a bit like a pimp wearing a fur coat).