At least one of the starters the Cardinals will add needs to be an ace-level pitcher. The lack of an ace is what has cost them in recent years. I don't think they'll be able to add both Nola and Snell via free agency.
I know his stats aren't great this year. He's 9-7 with a 4.43 ERA in his 22 starts. But what is most enticing about Nola is his ability to strike hitters out. He has 139 strikeouts on the season and is currently averaging over nine strikeouts per nine innings pitched.
He also pitched a gem against the Cardinals in the Wild Card round last October. He pitched into the seventh inning for the Phillies and held the Cardinals scoreless.