I think “crunch” can refer to a lot of things, which can present themselves differently. Having to roll three different sets of dice and do complex math every time you do something is one type of “crunch”. In another game, you always roll 2d6, so it’s easy-peasy, but there is a 300-page book of these 2d6 tables - a completely different form of “crunch”.
Mechanics and rules can do a lot of things, from rewarding mastery (and note that, even if the mastery doesn’t help you “win”, it can still carry social acclaim or status, develop a shared “language” for a group, building social unity, or it can simply be intrinsically rewarding - I recently read about a D&D group that was proud of how quick they were at getting through combat rounds, for instance) to satisfying a need to explore options, create formulae for moments in play, and so on… but they also have a real, performative aspect, based on what they make the people at the table do or say. Consider how the call of “roll initiative!” becomes a little ritual at a D&D table - it is a familiar call to action that gets everyone in a very specific kind of mood or mental space. It’s a reminder of both danger and the possibility of triumph. It has ritualistic meaning, for that group.
Simply rolling the dice or consulting a book can lend weight to a moment, just as much as it can ruin flow, disrupting a moment. I often think of how Monsterhearts’ moves sometimes add dramatic emphasis to small moments - a word spoken with a meaningful subtext becomes a ‘shut someone down’ roll, and an arched eyebrow might qualify for ‘turn someone on’ in the right circumstances. When and where and how is often a subjective matter of taste, but it’s worthwhile to think of the undeniable, objective nature of certain moves and certain designs, and how they might fit into your game or your game interests.
@DeReel, in the Forge days, the concept of “crunch unit” you are discussing was dealt with in some detail: it was called ‘Points of Contact’.
Points of Contact
The steps of rules-consultation, either in the text or internally, per unit of established imaginary content. This is not the same as the long-standing debate between Rules-light and Rules-heavy systems; either low or high Points of Contact systems can rely on strict rules.
A game could be categorized as “high points of contact, rules-lite”, for example. If you remember Jay/Silmenume, his D&D group rolls the dice multiple times a minute, but they are almost always simply interpreted loosely on the fly.