To follow on a bit from my post just upthread, I don't think that these are "siloed roles". I mean, Conan is a ranger (say, Beyond the Black River) and a burglar (say, Tower of the Elephant) and a warrior (in most of REH's stories) all in one might-thewed package! At best I would say that these works of fiction have had a big enough impact that they've established some received tropes or archetypes that people are happy enough to work with.
In the context of D&D, I see "class" - at least originally - as a way of integrating (i) the game-play demands of an asymmetric wargame and (ii) the overlay of fantasy colour. Chainmail already had it - Heroes and Superheroes as one type of unit, Wizards as a different type of unit that is more artillery-esque with a hint also of special forces - and D&D picks it up and runs with it. I find that Gygax's PHB is an especially useful guide to his thinking, and he describes class like this (pp 19):
Character class refers to the profession of the player character. The approach you wish to take to the game, how you believe you can most successfully meet the challenges which it poses, and which role you desire to play are dictated by character class (or multi-class).
This passage goes on to describe the functions of each class. That is reiterated on p 106 under the heading "Experience", and that later discussion concludes by saying that:
If characters gain treasure by pursuit of their major aims, then they are generally entitled to a full share of earned experience points awarded by the DM.
This is also one of those places where there is inconsistency between Gygax's PHB and DMG: in the latter book, rather than conformity with the functions/aims of one's class affecting XP directly, rather it affects training time and hence training cost (p 86). But the idea that class is about aim/function/approach to play is still present (with alignment now also factored in):
Consider the natural functions of each class of character. Consider also the professed alignment of each character. Briefly assess the performance of each character after an adventure. Did he or she perform basically in the character of his or her class? Were his or her actions in keeping with his or her professed alignment? Mentally classify the overall performance as:
E - Excellent, few deviations from norm = 1
S- Superior, deviations minimal but noted =2
F - Foir performance, more norm than deviations =3
P- Poor showing with aberrant behavior =4
Clerics who refuse to help and heal or do not remain faithful to their deity, fighters who hang bock from combat or attempt to steal, or fail to boldly lead, magic-users who seek to engage in melee or ignore magic items they could employ in crucial situations, thieves who boldly engage in frontal attacks or refrain from acquisition of an extra bit of treasure when the opportunity presents itself, "cautious" characters who do not pull their own weight - these are all clear examples of a POOR rating.
Award experience points normally. When each character is given his or her total, also give them an alphabetic rating - E, S, F, or P. When a character's total experience points indicate eligibility for an advancement in level, use the alphabetic assessment to assign equal weight to the behavior of the character during each separate adventure - regardless of how many or how few experience points were gained in each. The resulting total is then divided by the number of entries (adventures) to come up with some number from 1 to 4. This number indicates the number of WEEKS the character must spend in study and/or training before he or she actually gains the benefits of the new level.
This is not just, or even primarily, class-as-trope. Nor is it class-as-skill-bundle. It's
class-as-game-piece-function, creating the context for assessing whether a player plays well or poorly.
The fact that this purpose of class is now basically gone from D&D, but that classes carry on as the core of the PC build mechanics - and that classes are now taken to be tropes in themselves, leading to debates that are in my view silly, like the difference between a cleric and a paladin (in AD&D they're the identical trope) or a nature cleric and a druid (in AD&D there are no true neutral nature clerics) - shows the strength of the original D&D design legacy.