Thanks for the amazing write-up.

Always happy to share the trivia I obsess over! 🙏

If it ends up entertaining anyone other than me, I count the time I waste reading about it as almost a positive for society 😂

Why did they need to be volunteers?

In the strictest sense, they didn't need to be. But conscripted folks tend to be more prickly about being kept for ~20 years under military discipline in far-off frontiers, and conscription systems require significant enforcement systems to prevent evasion, especially if the recruitment pool doesn't feel an existential threat from whatever war is going on at the moment. Notably, a few legions were conscripted very early on in the Empire, and, not coincidentally, ended with one of the few mutinies of the Principate era, killing most of their officers, almost killing the Emperor's heir, and needing to be promised an early release from service to be mollified.

It's much easier to scrape up volunteers who come to you and are likely to not have significant prospects outside of volunteering for a long term of military service. They'll be more motivated, happier about a steady paycheck than the prospect of going home (where nothing worthwhile probably awaits them), and whatever retirement bonus offered will likely keep them on roughly good behavior until the end of their term instead of doing something silly like mutiny, or desertion.

Eventually, the Empire, over the course of the disastrous Crisis of the Third Century, does shift back to conscription. But the entire Legion system falls apart around that time anyway, and the Late Empire constantly fights with evasion of conscription and manpower shortages despite having a military of a similar size to the mostly-volunteer Principate.