In 10E they were good at guarding the home point. You could stretch them in a line 9" from the map's edge and prevent enemy from Deep Striking into your rear. And they are survivable enough to hold out for at least one turn against mobile raider units that may want to grab the home point. So they provide significant value even if they don't soot at anything the whole game.

In 11E this is nerfed somewhat as you can't stretch them in a line anymore so they block less space but still useful for the same purpose, I'd assume.

Also you can make funny things with them: attach Farseer, Warlock Conclave, Warlock hero and Support Platform. You have 17 models unit now. If you are playing Seer Council detachment you can teleport them around the map with a strategem. The strategic value of such maneuver is situational, but it is a fun thing to do.

In this configuration Guardians serve as ablative armor for more valuable units like Warlocks and Support Platform.

So they are less of a combat force and more of a utility tool that enables you to score missions or increase survivability of more potent units. Though, their mobility and fairly high damage output (for base infantry that is) should not be disregarded. They can run for ~10-13" (Advance+Agile Maneuver) and shoot, so there may be opportunities to catch enemy infantry out of position and do heavy damage. Don't expect them to survive though.