So you know how in the midgame when your team is sieging mid as 5...
and the enemy team is sieging mid as 5...
And the game's basically turned into aram for 10 minutes...
and there are those sidelanes, that nobody (in bronze) goes to clear?
well, with the new minion changes, the team with a level advantage will have their minions slow push harder than the enemy minions (pushing slightly harder if they have a turret advantage too, scaling based on number of turrets taken over their enemy). Note that the minion changes only affect minion damage dealt and damage received against other minions- they are just as easy to waveclear/push out as they were before. What's changed is that the creeps will naturally fight closer to the losing team's towers.
As a result, the team that's behind get's to farm up harder to make a comeback.
and the winning team get's the opportunity to siege more often, or otherwise risk the enemy team catching up.
Interestingly enough, in Dota 2, when the Barracks (the Dota 2 equivalent of an inhibitor) are destroyed, all the minions in that specific lane get stronger and they give less gold/exp on kill. This is because the lane autopushing is actually a huge advantage for the team getting pushed into as it allows them to farm up in the safety of their base. Dota 2 acknowledges this by decreasing the gold and exp values so that the losing team doesn't gain an advantage for losing an objective.