Wearing and wielding the right stuff is an important strategic factor in Rogue and onwards.
This is really more of a discussion between intrinsic and extrinsic improvements to the character. Intrinsic benefits are the kind given through level-up, and extrinsic benefits are the kind that the player is granted through various items and equipment: more generally, intrinsics are permanent (or at least difficult to gain/remove) and extrinsics are not. I would argue that this sort of character-building is a consequence of the heavy use of procedural content generation, rather than inherently designed. This is to say that, if you want to make the player forcibly adapt to the situation, they will be able to do so more fluidly if they can keep what they want and get rid of what they don't want. Skills and feats and (especially) character classes work in the opposite direction by constraining the player to a particular style of play, which can be interesting to a point but tends to lessen the factor of replayability as a whole.
Naturally this isn't always the case. for instance, Rogue Survivor's intrinsic gains (skills) are wildly dependent on the extrinsics (items) available, and so the player can compensate "bad rolls" with some padding where it's needed. MageGuild is similar to this extent and, while the player's spell choices are going to be important when it comes to having a particular fallback constant, it's the items that make or break a given game. Unfortunately, however, combining the two often leads to choosing extrinsics as a result of intrinsics (e.g., only using maces because you've already invested so much into mace expertise), which is why the focus of roguelikes tends to be toward item gains and away from leveling up.
Diablo (and probably even moreso in its sequels) chose to focus on items, although I would say this is done in a way that isn't all that meaningful. The "good" from that game are the unique items, which literally have some unique qualities that stand out from any other item and add some depth to the strategic and tactical decisions. The "bad", on the other hand, are all the other items that players only have to min-max to figure out what's good and what isn't. The items may as well be intrinsic-like, since you only get rid of one for something that is quantitatively better: in many ways this is the same thing as leveling up, except that the achievement condition depends on what enemies drops and not the killings themselves.
That's really all I have to say. I think in an environment where so very many games have the idea that the player's avatar should always grow exponentially without fail, it's nice to see roguelikes expect the player to adapt to stay at the top of their game.
