Eras done for balance purposes?

[Chazar]

Eras are good: I recall that I read somewhere that the Eras also influence the number and kind of magic sites, etc.

In Dom2, the MP-community seemed pretty obsessed with a certain single standard setting and convincing people to different setup was difficult.

With three standard setups (aka eras) this will certainly be more flexible and simplify the selection of suitable opponent players to fit ones own game style preference...

Somehow people are reluctant to deviate from the norm (like using alien mods). So making everyone choose is good.