Blockchain.com wasn't the one who built the first blockexplorer. You are right; no one actually needed to see the balance of someone else's address and most of the information that you see on the blockexplorers are not essential to most.

The first blockexplorer was actually created by theymos: https://bitcointalk.org/index.php?topic=1727.0. This was before blockexplorer.com and it led to the creation of that which was a lot more polished and with more detailed information.