Archetypes/professions/classes are also tied to the concepts around them, making it easier to communicate between the players and the GM (and even the community). If one speaks about the "Warlock", everyone knows what it is; or has a good idea about it. Presenting a list of archetypes/professions/classes to new players make it easy for them to know which character or kind of character they'd like to play.
In a pure skill only system, it's hard to communicate about a character concept except through intensive descriptions of the skills in which he'd be good and bad... and quickly enough, people would assign it a word to mean "that concept of a character being proficient in such and such skill sets, and bad in such and such skill sets"... meaning, a class/profession.
I mean, really, if you develop your skills in order to be efficient in combat more than anywhere else, wouldn't you call/present yourself as "a fighter"? Whilst calling yourself a "mage" if your focus is more on magic? Then, since there are thousands spell lists, as a need to differentiate each one, depending on their focus, would arise, people would naturally assign a name to each concept/focus; for instance, "elementalist" for the mage who specializes in manipulating elements. And even further, such as "elementalist" for the mage who specializes in manipulating elements, with no dominance of one on the others, "fire mage" for the one specializing in fire, "wood mage" for the one specializing in wood, "metal mage" for the one specializing in metal, etc.
For my part, I have an extensive use of professions, as "a set of development costs for skills and, possibly, a set of spell lists, based on a character concept". It's very useful to me and my players (old and new), because it provides a common framework of skill tweaks (and spell lists combinations) that were proven to work (more or less).
1. It makes it easier for a player to know from where to start, without the burden to compose himself his character (e.g. spending hours tweaking the skill costs),
2. It makes it easier for a new player to start, since he doesn't need to know much of the system to understand what kind of character he'd have if he takes a "Thief", or an "Archer".
3. It makes it easier for me to remember a NPC, out of my thousand ones, and what his proficiencies are (just as, IRL, one would remember more easily "John Smith the Lawyer in Paris" than "John Smith of Paris").