As the title says, why are the surname and patronymic formed from Яков Яковлев and Яковлевич instead of *Яковов and *Яковович? Where did this -л- come from? Is it simply because Яковлев(ич) is more euphonious than *Яковов(ич)? If so, аre there other examples where words or names formed from a root add an -л- through the same process by which it was added to Яков?
Also, am I correct in thinking -л- was chosen for the same reasons -л- shows up in certain first-person verb conjugations, i.e. оставить --> оставлю?