This is part of the differentiation effort for the romans.
This is a very unique technology that trades in melee CS infantry in exchange for unlocking legionaries. Legionaries are similar to skiritai except they are cheaper.
The idea is that these units can form the bulk of the roman army, so they should be cheap, but powerful. Additional champions remain to supplement these units.
In addition, for improved engagement with centurions, centurions gain a special second aura just for legionaries, called 'Century Maneuvers"
To do: