application of key usage extension

application of key usage extension

I understand how Key Usage Extension of x.509 certificate works. I have gone through the Key Usage section of RFC5280 and I know of all the valid values and what they mean.

But what I don't understand is the usage; Why it was deem necessary to add Key Usage extension to x.509 certificate? Is it solving any problem or is it used to prevent some obscure attack?

thanks