Here's one usage. I was playing as Genoa from 1399. I wanted a solid Black Sea Colony, so I went to grabbing eight total provinces. First I converted to Catholicism, then started settlement policies. I was obviously not going to be using my colonists elsewhere, so no big deal on their loss. I started with Crimea, and that converted in about 5 years (I was lucky). I haven't yet gotten to any others, but since it converted well before 1450, I'll definitely benefit from it long term (assuming I can hold the Crimea).