Simon’s name was changed to Peter (Mat. 16:18) because Jesus wanted to highlight the important role Peter would have in the early church.
Peter means ‘rock’, so when Jesus said ‘on this rock I will build my church’ (Mat. 16:18) he was using a pun to refer to Peter’s role as one of the strong, supportive leaders (a “strong foundation”, so to speak) of the early church. We can see Peter’s important involvement in the early church in the early chapters of Acts.