Having allies is easier than having subjects.
“By contrast, the socii recruit themselves, equip themselves and organize themselves. And they fight hard, because they’re fighting for their home communities, to show their bravery to their own neighbors and family (who are there with them) and for an equal share in the proceeds of victory. While the troops provided by subordinated peoples were often the weak link in Achaemenid or Seleucid armies, the Romans felt perfectly free to use socii as they would use the legions. Polybius makes almost no distinction between them, whereas Livy in his battle narratives often stresses the valor of the socii.”
https://acoup.blog/2023/10/20/collections-how-to-roman-republic-101-addenda-the-socii/