Because every unit always is alone on its own...
There are a lot of options in a lot of books that are medicore at best when only used on their own but get way stronger through synergies.
A properly used (and that means Detachment) Guardian Squad comes at 90 points, always runs 6" and gets a free heavy weapon (with relentless) ranging from a skyfire Missile launcher down to another multi shot weapon with Bladestorm. For another 35 points it can sit down and enjoy a 3+ or even 2+ cover save and probably another minor benefit.
Or you just add a Serpent (which is still good and the only reason why the other Aspects are good - or would you argue that footslogging firedrakes are any good?) and get them close to blast away.
While they are surely not anywhere near the top ranking of all armies they are still a decent choice. It is just that DA and Windriders are better. The former because of their galore of special rules, the later because they are extremely undercosted.