Hallowed Seeds

Buying these from the Dryad is how you put Hallow where you want it instead of waiting for it to crawl there on its own. The usual reasons are a fishing spot near home, a farm for Hallow-only enemy drops, or a Pearlstone supply that is not half a world away.

The restriction that catches people is the surface. Grass seed only takes on dirt, so throwing them at bare cavern stone does nothing at all. Lay a dirt floor first and plant into that. Once the grass has a foothold the biome converts the stone around it without any further help from you.

It works as defense too. A deliberate block of Hallow placed against advancing Corruption or Crimson is one of the cheaper ways to slow that side down while you deal with something else.