The Guardians are the ground forces of the Empire.
Why does an Empire bristling with super weapons capable of cracking planets open like eggs need an infantry? Because the Infantry Soldier is, and always will be, the ultimate weapon: You dont use a meat cleaver to cut out a tumour you use a scalpel.
The average Guardian is far from this subtle but shes the best the Empress can do. As most wars in the Empire are small counter insurgency affairs Guardians are taught to make the best of their shoddy equipment and fight like tigers whatever nasty scrape they fall into.
Tactically Guardians are about outshooting their enemies. All of their Favoured Weapons are Medium to long range and they gain an additional bonus for using them making them a really nasty bunch of heavily armed battle bitches.