Matching up grants significant stat boosts, lets you conceal squishy units behind stronger ones, and just breaks the game in half in general. Unpromoted classes get abilities at L1 and 10, promoted classes at L5 and 15 (fire emblem three houses silverfish). Master Seals are used to promote systems into a sophisticated class provided they are at least L10, and are infinitely buyable after Chapter 12. However player A can be attacked by the opponent system on its stage and lose their character, while gamer B has the opportunity to strike initially and win combat. Or, at the minimum, escape and heal. fire emblem three houses routes. See how much safer it can be to let enemies assault you when you're not ensured to get a kill? Don't be scared to use promoted systems! Fire Emblem video games typically start you off with a strong system that gains a lot less experience than other systems and has shittier level-ups, and a typical reflex is to hide them in a corner of the map to make certain they'll never ever steal precious experience points.
Why not let them catch a grenade for you? Toss their heads on a blade for you? Dive in front of a train for you? You understand they 'd do anything for you! Take decisions while bearing in mind the worst case circumstance. Are you truly happy to take a 45% chance that the unit you want to train dies? Maybe it's a much better idea to let a more powerful unit look after this one (fire emblem three houses ashe).
Close ranged units can kill squishy characters like mages or specifically archers, long variety characters can wait till an enemy is close to death and finish them off without ever being in threat (fire emblem three houses fishing). Enemies that can assault a minimum of one character will always move and attack this turn unless they're not able to relocate basic.