Silver 2P E+ seat selection question

I am a silver and I have a upcoming trip SFO-ICN-SFO.

Based on my experience, not many 2P 1P Platinum and 1K members on this route.

I am not planning to purchase E+. I am expecting free E+ at the time of check-in.

However, if I select E+ right now, UA does not ask me to pay the E+ price. If this is the case, what is the difference between selecting E+ at OLCI and selecting well before?

Thanks,