Caesarean Legionnaires without armor?

[Tarbicus]

This may be a bit off-topic but it's a simple question:
I've seen in several pics that roman tunics have a knott in the back (just as shown at the second pic of Graham's post), why did they do that?

The neckhole was very wide so that, when doing manual labour or it was hot weather (?), the tunic could be pulled down below the right arm. When the neckhole needed to be tighter a knot was tied to keep the material gathered.