Midlands Connect say HS2 will improve services to 73 stations, 53 of which aren't connected to HS2 [1]
It also introduces capacity gains for commuter lines into Euston (can't find the link for that)
I'd encourage everyone to read Rail Magazine's take on why we should build H2S [2]
1. https://www.midlandsconnect.uk/publications/hs2-released-cap...
2. https://www.railmagazine.com/research-hub/comment/comment-sp...