i just dont know and wonder what is the benefit if we use an avatar?
None!
Everyone could have avatars in the next update of bitcointalk.
There are no benefits with avatars.
That is not true. Having an avatar means that you have some level of prestige and avatars are somewhat coveted by people who have actual interest in the forum and in bitcoin. You are however correct that they do not allow you to earn any additional amounts in signature deals
so how do we get to setting an avatar? or maybe only for certain members
You had to have had an avatar as of prior to when the forum was hacked in late 2013 (the attacker was able to hack the forum via the avatar system).
As a result of the hack the changing of avatars (including the adding of one) is disabled, although you can PM theymos to have your avatar removed (but he will not add one)
But can't the attacker simply use the members that currently have avatars to attack the forum?