I imagine DC buys rights to other companies' characters, not because they expect to make good use of them, but to keep them out of others' possible use. There's a chance some little publisher who gets rights to the MLJ heroes or the Charlton characters could conceivably have put out a line of comics which would draw sales away from DC... especially if a hot new artist was involved. Assimilating them into DC prevents that.
The good side of this is that quality archive editions are available that may not be printed otherwise. The bad result is that the T.H.U.N.D.E.R. Agents (for example) will just show up in a few panels of the latest cosmic slaughter to be killed.