Preferential means that you do not get much stronger opponents than you, mostly same size or weaker. Easier to win. Some of them are good some bad. You buy one that you can use for crew training for tank you have or plan to have. It is nice to have few to help you get credits.

What is preferential matchmaking in world of tanks?

What is a preferential matchmaking tank? A pref.mm tank is a tank which gives up certain advantages that their tier usually would have. For instance, the Tier 8 Russian heavy, the IS-6, has an atrocious canon, but that is the trade to get pref.mm.