The Tactical Command Section is a group of models that facilitate the rapid dissemination of commands throughout the corpus of the army. In the absence of a Lord Marshal or Legate Commander, the Strategos of the Command Section functions as the Warlord for the army.
Arguably, their strengths lies in the fact that they are an array of individuals, each with their own boons.
The Strategos carries the useful cognis-signum and can also call down a precision bombardment. For the points cost of the command section, this alone is probably worth it.
The Vexilarius carries the important standard of the army which means that casualties can be ignored for the sake of Ld tests when they have to be taken within a certain radius. This is incredible for an army like the Solar Auxillia. It alone also makes the Command Section worth taking even if the army also includes a Lord Marchal or Legate Commander.
The Proclaimator carries another useful device in the shape of a Nuncio-Vox.
Fundamentally, these guys are mortals and won't hold up to an Astartes assault. Equally, that's probably not why you're buying them. Therefore to counter the threat of Space Marines chopping their heads off in short order, the controlling player will undoubtably want to purchase one of the dedicated transports for this group.
There are a few build, but fundamentally only the option on the Strategos and the Veterans make any difference. Here are a pair of builds to mull over.
1 Strategos with a power axe, 1 Proclaimator, 1 Vexilarius, 2 Veterans each with a plasma gun (115 points)
This is what I would use as a baseline. The power axe is for challenges (in the unlikely event that the Strategos survives an assault down to I=1), and the plasma guns are there to take care of higher toughness and better saves enemies.
1 Strategos, 1 Proclaimator, 1 Vexilarius, 7 Veterans including 2 melta guns (120 points)
A larger squad here, armed with two melta guns which are meant as a threat to enemy vehicles should the opportunity a