Professions used here has synonymous meaning as templates. GURPS uses “templates” as a set of trait choices within a basic character idea.
The player usually must take a few traits which form a foundation for that character concept. In fantasy, a wizard would have the Magery advantage allowing them to have access to surrounding mana (power of magic) that characters without Magery would not. Other than how many levels of Magery, the player could not opt to reject that advantage in choosing a wizard template.
The player would, however, have plenty of flexibility in spell choices. Usually given a pool of points for spell selection in the wizard template, the player could choose to take many spells or have a few spells they can use with more skill competence than the rest by adding more points to those few spells and only one or two points to the rest of the spells they choose.
Templates not only help assure that players choosing a certain character concept have a basic foundation. They ensure players have characters rounded out enough to operate normally or effectively in the setting — traits and skills almost everyone would have. They do not always require the exact same skill, but they would offer a choice between similar skill types which achieve comparable competence during some interaction with the world.