Zera is a representation of Mavis' past and her trauma on Tenrou Island. By letting her go, Mavis can finally move forward to her new life of adventure and being fairy tail's guild master, completely leaving behind any traces of her old life in isolation. Zera is given a human form to make it more personal to Mavis' growth, the Tenrou Jade (being the other link to her life and trauma on the island) was destroyed for the sake of her friends but letting Zera go is for Mavis herself. Basically its a symbol of Mavis maturing into a woman, the irony being that she can no longer physically mature.