I suggest to think the opposite things of "本番".
The possible opposite words are like below.
- training (トレーニング)
- drilling (訓練)
- practice (練習)
- rehearsal (リハーサル)
- testing (テスト)
As you may notice, these actions are not the real purpose of you generally. They are kind of acts toward a big goal. We call this goal "本番" in Japanese.
Generally speaking, we don't expect failure in "本番" situation but failure is common in "練習".
In some kind of situations like presentations at a conference, matches in a competition, recital concerts, and examinations of entrance, "本番" means such events, especially emphasizing a message like 'This is not practice!! You have been practicing for this!!'.
This is the original and most basic usage of "本番" in my understanding.
I think "本番" is basically an event like above, but it is very useful word to tell a situation like 'This is not practice!!'. So it's used in wide cases.
「だってIT社長って、女優と付き合ってからが本番でしょ?」
This indicates that the life of IT president who doesn't date an actress is NOT "本番".
In other words, the life of such a president is kind of 'practice'. (even though every life should be "本番" because no one would expect failure of his/her life) Therefore this serifline leads us to cynical impression.