How gem inserting can be useful?

[Kadarus]

There are two NPCs that allow you to insert a gem into a blank ring of an appropriate tier. Also Alchemist can insert a ring into an armor (belt/helm if you go for that prodigy).
However, the rings/armor of the same tier that you can find in the dungeons are almost always better than what you can get with gems. Lifebinding Emerald is probably the only exception here. So why would you use that ability on other gems?

[HousePet]

Alchemists can insert gems into any body slot armour, not just plain ones.

[Danton]

There are other unique gems that are also very good in their own right. A Goedelath Rock in a Corrupted Gaze makes for one hell of a corruptor helmet, for instance, and getting two uniques in a single amulet (after that one quest) is very strong.

[Kadarus]

Well, if alchemist can insert gems into any body armor, even with ego, that changes it, thanks.

[ghostbuster]

And even in artifacts.

[Atarlost]

The jewelers remain pretty much pointless, though.

[OrtRestave]

One case where the jewelers aren't so bad is if you're lacking stat boost jewelry and your build depends on it for prereqs.