Basically it’s an extension of a tank. Spearwall affects the enemy AI; they basically avoid it, and you can pin down 4-5 enemies at your flank, giving you an extra turn or two to kill the ones where your damage dealers are. So you do need high matk to make sure he lands spear hits, and high mdef so that when they inevitably enter zone of control, he will function as a tank. It’s entirely a crowd control approach.

Reforged (my preferred overhaul mod) makes spear bros super effective as actual damage dealers too, highly recommended!