(in the US) So that the government can conduct a background check on you to determine if you are legally qualified to purchase a firearm.
A private individual does not need a license of any sort to purchase a firearm from another private individual under federal regulations administered by BATF&E. State laws will vary, with some states requiring a permit to purchase a handgun.
Only when a private individual purchases a firearm from a Federally Licensed Firearm's dealer, is the dealer required to conduct a background check, called a NICs check, to verify that there is no record of the individual being legally prohibited from possessing a firearm.
yes
No it's not necessary.
No.
no
no
yes, you just need a license
Generally, no.
No.
No ... but their may be an age requirement.
Yes.
18 but if you are under 18 but over 12 you can have a minor's license where you can't purchase a gun but you can use one and buy ammunition.
It would depend on what state you live in ... but in most states you would not need a license ot own a softair gun ...