Simply put, we could use all the good blocks when designing builds meant for worldgen without breaking the balance of the game in the process. Fancy quartz towers like the ones I designed for Ruins mod could blanket the landscape safely without worry that they would be used on tools for Tinkers Construct.
Blocks could have a "Crumbling" NBT tag, they could break faster, have an alternate break animation, emit particles while breaking, and drop nothing when broken, save for a cloud of alternate dust particles.
Mods shouldn't ever be used as a reason to add something, plus it is not possible to add NBT tags to blocks (placed in the world) because they do not support it (many blocks would not support new data values either because there isn't enough space, plus there is no such thing as data values/metadata in 1.13 and later - every state a block can be in, including formerly render-only states like fence connections, is actually a separate block); you'd need to add entirely new blocks and in that case it would be better to mod in a new block that looks like the one that you want to use (of course, being able to actually add new blocks to vanilla would be a major thing).
The only other way this would work is to use a tile entity which could assume the properties of the block that it represents, which is more practical than it sounds (tile entities are often cited for causing lag but that is mainly because they either require updates every tick and/or use a special renderer which renders them like entities, which are extremely expensive compared to blocks since the game renders them every frame instead of just when a chunk is updated, which also batches all draw commands, minimizing the number of OpenGL calls. As an example, I modded flowerpots to be tile entities (they were normal blocks prior to 1.7) and even thousands have no major effect on performance (other than increased memory usage, but there were 112896 tile entities loaded and you likely aren't going to use that many), even when they are rendering miniature models of actual trees with the equivalent of up to several thousand blocks each, unless many are in a chunk and it is updated, since they are not ticked and are rendered as a normal block).
Mojang isn't likely to see this anyway, still it may be useful in some cases, of course there are many things that are more useful in game, and describing the attributes is the first step, even if it may take until minecraft version 2.37, people have to start thinking about it for it to eventually make it in.