What exactly are mate threads for?

On the old forums, people would make mate threads to find their mate, basically to some people it was
a. best forum friend
b. a forum relationship

so being someones mate usually mean you talk to them a lot && roleplay together.

As stated above. ^^

Mate threads is just another fancy word for Forum Best Friend / Roleplay Partner.
It helps expand on the lore of the Warriors series.

Just like in a roleplay you have mates, which consist of different role-players creating relationships with various created characters. A forum Mate is that supposed one person on these forums that you can consider your always standing by your side "role-play partner."
>>>> Gamers terms: "A static group to do static runs"

To keep it to the lore, that role-play partner is just called your mate.

This doesn't mean that you guys have to create characters who are mated together ALL the timee. It just indicates that at the end of the day, you will have that one user you can role-play with when bored, to talk to, or for both of you to join a role-play together with a somewhat established story of your two characters.

Forum mates are not necessary, it's just to keep with lore, or to have someone to always rp with on the side when absolutely out of it. Usually these mate threads are to find candidates of users who fit your personal style, interests and fandoms.
(This way you guys can role-play together in more then one genre whether it be Warriors or Off Topic)

As said above, it is not a serious thing. Some do consider it like a forum relationship, but it doesn't have to be.
Just a forum BFF, to talk to, and role-play with.