DraStic

Some DS games can have special functions depending on the GBA game and their save games.

Example:
-Pokemon Diamond can import pokemon from Pokemon Emerald
-Megaman Battle Network 5 DS changes music and forms depending on the GBA MMBN games (or MMBN5 GBA save game)
-Lunar Knight have bonus with Boktai series

Not sure if there are others.

Use cheat. What else is different with cheat and gba slot things?

Well for one thing, you can't get that moogle from fftactics a to fftactics a2 without gba save.

There are usually cheats to replicate those effects or receive the same thing without the need for a GBA slot also not to seem heartless and cruel but there is no real need for it since most games on the DS don't even use the GBA slot.

Touche there but then again, suggestions aren't that bad even if it is redundant.