>>716420462The idea is that you have your attackers at the far left as your last line of defense instead of sun plants and that sun plants (at least sunflowers and tiny sunshrooms) are more replaceable than your attacker plants due to being cheaper.
The first reason doesn't really play that much of a role in PVZ1 because it's not that hard, but it does play more of a role in modded challenges or PVZ2 or its modded versions due to higher difficulty as you're more likely to see zombies bypass the frontlines more.