They want to find Wastelanders to drag back to Vault 87 where they will dip them in vats of FEV and turn them into more super mutants, the way the Master used to.
They only don't have a long-term plan, like the Master did.
They just want more.
They're looking for victims, but also a new source of FEV, since the vault is running out. You can overhear them talking about it sometimes in Vernon Square and the Germantown PD station - they're looking for more "Green Stuff" to make more mutants.