Except for giving themselves an overwhelming advantage in literally every aspect of the game OTHER than getting free pvp wins by virtue of just having paid more.
Why would anyone care about any other aspect of the game if it didn't affect PvP??
PvE will continue to be your F2P mode where everything can be solved with an Excel Spreadsheet. All the numbers are known, all the teams are known, everything is just "hit minimum stats to win" and provides zero long term challenge.
What PvE does though, just like shop and shard purchasing, is provide the champs and materials needed to compete in PvP. That's what it is here for.
PvP is the endgame in which everyone is driving to. There would be zero reason to build a strong account, through either purchasing or farming or both, without the endgame crucible that is PvP. That is why you get shards. That is why you get gear. That is why you build champs way above the requirements any PvE challenge could require. For Arena, for Ceeg.