They even have list of all bitcoin addresses ever generated by some services like yobit, bitpay, xapo etc because all of them generate addresses with their one HD wallet for their users.
I just come to know about this, I guess it will be more useful for tracing out some transactions like where it will be ending up.
I know about walletexploerer as it is being used by forum members to find out alt accounts but now I come to know about the other most useful feature of walletexploerers as I need to find out some people who might have ended up their bitcoins into exchanges after that I know how to get their username/email from exchanges.
Why would you want to know if someone sent bitcoins to some addresses? any exchange service giving away their clients information please let us know as well so we can avoid registering and using their services.
This is exactly where a mixer service can be used to mislead those people trying to link some addresses to you.
Also if someone using address generators such as bitaddress.org then there is no wallet to look for at all.