Take a commoner who’s more physically fit than average, put a spear in his or her hand, give him or her some combat training, and you have a guard, the first line of defense against player characters who might otherwise run amok through the picturesque towns of your campaign setting.
With their above-average (though not exceptionally so) physical ability scores, guards are well suited for the simple, direct combat role of “Go ye forth and poke it, then poke it again.” That being said, they are essentially nothing more than well-trained commoners. Braver than average they are, and more motivated by duty, but this presumes that 99 times out of 100 they’re not facing any foe more challenging than another commoner. Throw a monster at them, and they’re as likely to flee or freeze as they are to stand and fight. They know how to use their weapons, but that’s not the same as understanding strategy and tactics; their sophistication extends only as far as knowing that a surrounded foe is less likely to get away, so if they outnumber their opponents, they’ll flank, and if they don’t, they’ll form a line, circle up with their backs to one another or send one of their number to run and get more guards. When they’re in serious danger—reduced to 4 hp or fewer, or next to another guard who is—their discipline dissolves, but not so much that they’ll run without Disengaging (action) first, unless they themselves are the ones seriously wounded. (The difference between their physical abilities and those of a commoner is significant enough that nonhuman guards will take the direct approach to combat even if commoners of the same race wouldn’t.)
Note that guards don’t even have proficiency in the Intimidation skill. They can yell, “Halt!” but it’s not going to make anyone take them more seriously.