Back when AK kits were cheap, matching numbers gave you a better chance of all the parts fitting together as they came off the same rifle. Akm rifles have a wide variety of tolerances, so unmatched parts may or may not readily fit. The matching numbers give you a better chance of the parts fitting together. But I have had factory assembled rifles that needed work (namely Russian Saiga-12) and unmatching parts that drop right in. So there isn't a certainty, but maybe the perceived higher chance of fit parts.
I've had arsenal force fit rifles that ran fine (Russian & Chinese SKSs) and everything fit together well. They had the old numbers stamped out and the new numbers marked (many times crudely). I've had some high end gunsmith built rifles where they removed all the mismatched numbers (welded over and ground down) and put together a really nice finished product. I've also had Century mix matched rifles with surplus parts and a new US barrel that ran great.
There is also preference. If I could get a matching or non-matching rifle for the same price, I would take the matching numbers. The question then becomes, how much more would you pay for a matching vs. non-matching rifle/kit. It isn't a big for a shooter than a collector. I move back and forth, but will take a deal for non-matching.