When the damage is beyond what a home stone can reasonably remove: chips you can see, a badly rolled or recurved edge, a broken tip, or a secondary bevel that has grown so wide the knife cuts poorly even when sharp. All of those need a lot of steel removed, which is a grinder job rather than a stone job.
Visible chips deeper than a hair. An edge so rolled that honing no longer straightens it. A broken tip that needs the profile reground. And a secondary bevel that has widened over years until the blade wedges in every cut.
Any of these means removing more steel than a pocket stone will remove in an evening.
Usually reprofiles on a belt grinder, which removes material quickly and evenly, then finishes by hand. That is genuinely difficult to replicate with hand tools and easy to get wrong with a bench grinder, which overheats and ruins the temper.
Some makers offer this themselves. Buck runs a paid sharpening service, which is separate from its free warranty repair.
A merely dull edge does not need a professional. Neither does a knife that has never been stropped. Both come back with twenty minutes and a stone.
And think twice before sending a collectible. See sharpening a collectible knife.