Some advanced advice:
Since the game works with the breakpoint system, which is almost always the same as your troop amount, but each troop contributes 2 when routed, you'll get roughly half of your troops routed before you lose(depending on your camp, scythed chariots, etc). Thus, it's better to always feed the better troops to the meatgrinder first for the reasons mentioned by the others. There are some exceptions, like if you are attacking pikemen with knights, I prefer to do it with Average rather than Superior, as I assume I'm going to lose(sacrificing them). Why would I do this?(normally you'd be crazy to do it, but sometimes you might gain an advantage) It may allow me to do a rear charge, or prevent them from retreating so I can get other troops in contact with them. Then, hopefully the knights will survive without routing(even if at low %), and I will run them to a safe place. It'd be a waste of superior unit to use such a unit for such a task. (Note, using hyper-aggressive tactics only work when you know what you're doing or are desperate, so you might not want to do it until you can assess the situation properly

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Sometimes, very rarely, it's to your advantage that your units rout easier and fight worse. Suppose a situation where you have many more BPs than the enemy. This likely means your troops are worse. If so, you should be careful about your better troops. It's usually worth sacrificing an Average troop(say, charging at the enemy, being routed, then the enemy chases after your unit to a position where you can rear charge him immediately) to get a better enemy unit down. The worse your unit, the more likely they'll lose and rout, thus the more likely your kamikaze plan will work. If the troop manages to win the fight, it may actually be worse, as it'll allow the enemy to reposition his troops to avoid such things from happening in the future and you've probably lost a unit without being able to rear charge the enemy. A risky style, but it works at times.
I always prefer using worse quality units for arrow fodder if possible. It usually is possible because I'm almost always the one attacking so I get to set the pace.

Thus, especially when fighting Drilled Bow-units, it's preferable to march your worse units to be shot at, with your better units behind them(armor affects this, though. I'd rather have Armored Superior units to be fodder than Protected Average, since they take less damage). Of course, using Skirmisher units like Light Foot is the best for arrow fodder(better mobility, never gonna melee anyway, etc). You just have to be careful to retreat your units well before they're close to Breakpoint, especially poor units. Going from 80% to 65% is fast, and losing a unit since you didn't retreat them in time is always painful. Disruptions are easier to avoid, since only one can happen per turn, your unit can't go from D to R due to arrowfire(from morale tests, it can from unit damage), so it's easier to manage your potential defeated units if you have to avoid disruptions rather than damage.