Welcome to the forum NomKod!

Many reasons, but principally:

1. LOD - in the strictest sense of the acronym (not the BIM sense) insomuch, some authors like/need to limit visible geometry "at scale" so employ a "bounding box" to show at COARSE view settings - so that, for example; at 1:500 or above, small (or even large) "things" only appear as a simple box - rather than show all the minutia (that's unnecessary at such a scale, and urgh, tends to become a mess of crushed lineweights)

2. COORDINATION - unless you're modelling a space ship, hyper car or data-centre, its rare you need to be able to route other elements "through" other elements (some exceptions, but still) so people will model the "bounding box" of a "thing" so that during CLASH AVOIDANCE tests (in Revit, Navis or elsewhere) there's no scope for "other things" intersecting (say under a table, or between the legs of a chair)

Because they are terrible content creators, basically.

But what i mean by that is reason 1, that Snowy posted: They make a giant box for Coarse (and sometimes Medium) LOD. Its damn awful, its also a waste, and imvho makes the families WORSE to deal with and more complicated than they need to be. One of my "content remediation" steps is to blow that cube away, and set the real geometry (well, most of it, if there are complex bits) to be visible at C/M/F.

That is sometimes dictated by the people who are paying for the content, but yes- it is a terrible practice and I hate it.

I really liked the keyboard example you used in some presentation I saw you give. All the geometry is there, at coarse the keys are not visible and you just see the shell, at fine you see all the keys (for say- a rendering.) Made it really easy to understand the concept 🤘

I still have those keyboards, hahaha.

There is some logic behind the different graphic levels:
Some countries have requirements that during early project phases the LOG (level of geometry) is kept down to 100/200 LOG. (The UK is one of these countries, and some of the Scandanavian countries also, also Luxembourg) I have no idea for the US market if this is the case.

So if you are getting your families from European or UK content creators that would explain why you see these as for example on bimobject website these different LOG controls are in fact mandatory to be included in content provided to the website for download from suppliers.

I have to be honest and say I've never seen those large "blocks" 🤣
Multiple geometries for detail level, sure / sometimes, but I usually avoid manufacturer's content like the plague anyway.

Strangely enough I'd never seen the block either until up about a week ago just before this thread was created, think the models were all sourced from a furniture supplier (who I'm assuming outsourced the family creation).

Having originally been brought up with the "different models for different detail levels" methodology, I can't begin to express how much better my life is since these forums showed me the light a few years ago.