Welcome to the org Taiwan Legion

My general impression is that guard mode will increase unit defense stats at the expense of a reduction in attack stats. The increased unit defense allows your line to hold for longer while you attempt flanking etc. It will also stop units chasing after the enemy and breaking up your line.

I tend to use guard mode on my main line and archers so that my main line will stay together. Then I will send my cavalry round the sides while my infantry is holding the line. Hopefully while my line is still holding I will charge my cavalry into the backs of the enemy and they will rout.

Hope this helps.