If you're battling somebody, but your opponent doesn't post in more than a week, then the victory will go to the person who posted last. If that makes any sense.
It's basically so every battle you get in you'll get some experience, since it can be hard for your Pokémon to gain levels otherwise. It might also encourage people to be active on the site >3> Keep them on their toes.
Example:
Poster A: PUMBLOOM USE JUDGEMENT
Poster B: OH NOOO PIKABLU USE V-CREATE
Poster A: PUMBLOOM STRIKE BACK WITH GUNK SHOT
Poster B: *never posts again*
VICTORY GOES TO POSTER A, GG, 5 STARS, A+, 10/10, PUMBLOOM GROWS FIFTY LEVELS
The end.