Hi & Welcome to the Forum!
In my opinion, the biggest reason for joining an alliance is for the titans.
Even low level players get more loot from joining an alliance and hitting the titans. They drop some of the ascension materials (sure very rare at low level titans, but still a chance). Also give gems & other such items which are not strictly farmable.
Initially, low levelled players may be something of a liability but by joining an alliance, they will grow more quickly both because of the knowledge they can gain AND because of the extra materials & items dropped by titans.
Click here for stuff about titan loot tiering, items received etc…
The main thing that many Training Alliances want is a member. Once the member is in & are active, they will grow and become a valuable member of their alliance 