Board Thread:Manga/@comment-5774380-20161007195718/@comment-27321453-20161009165140

King Fritz didn't need to control those mindless Titans the Marleyans keep dumping onto Paradi because he had the millions Colossal-like ones inside the Walls. That's what he threatened the Marleyans with 80 years ago.

As for why they transform them instead of executing them, being trapped inside mindless Titan forms is like having a nightmare that wouldn't end (as Ymir described it) so it's a fate worse than death.

Plus they're forced to eat their fellow Eldians in the SC branch and act as eternal siege of the Walls. So it's worth the trouble of transporting and injecting them with Titan serum considering the benefits.