Plenty to someone who wants some crunch and doesn't have the time to create and playtest it. Or to the many thousands of D&D players who don't have the old books and want, you know, books.
Its not even
just the crunch. It is a chance to adjudicate (whether we like their adjudication results or not) all of the inconsistencies across different published products (i.e. the canon) for the campaign setting.
Just as a very simple example:
1e Greyhawk Folio states that the Kingdom of Keoland's King Tavish III was killed in battle in 453CY. With Tavish IV succeeding him. They mention the king is a lvl 14 Ranger. Ok. Who is the current king since it is now 576CY? The subsequent boxed set identifies the king as Kimbertos Skotti, but no info on his class or level.
From the Ashes doesn't provide any of this info other than that Kimbertos Skotti is the king.
I don't have Greyhawk the Adventure Begins, so I'll skip to the 3e LGG (if anything I mention in 3e LGG is in GtAB, that's on me).
We get a lot more info in the 3e LGG - that a regent rules until 414CY. Ok
that's new info - no mention of a regent until now. Now we also know Kimbertos Skotti ascended the throne in ~565CY. But, he is now listed as a lvl 15 Ranger. we also get info on the local barons in each of Keoland's provinces (with stats).
So - why is it important to adjudicate any discrepancies? Well - an AD&D Rgr14 gets 3/2 attacks per round and a Rgr15 gets 2/round. In 3e, a 15th level ranger gets access to 4th level spells beyond bonus spells for a high WIS score, an additional favored enemy, and bonuses against one of his favored enemies increases.
So - which do we convert to 5e? The AD&D 1e Rgr14 or the 3e Rgr15? Its the difference of one additional ranger archetype feature and one additional 4th level spell. That feature is actually pretty good - evasion, uncanny dodge or stand against the tide - something that a 14th level ranger doesn't have.
Does this difference matter? For some people, the difference between a Rgr14 and a Rgr15 may be, in their home campaign, of significance.
WotC's conversion document says that for 1e-3.5 stuff, class levels convert directly (up to 20th level). So - do we convert the 1e Rgr14 or the 3e Rgr15?
And - this is a simple example from a not-all-that-detailed set of 1e-3e info. Imagine a DM trying to sort out stats and lore for the far more complex Forgotten Realms... It can quickly become overwhelming.
It's that sense of being overwhelmed that, for me at least, means I'd prefer to have an adjudicated set of info.