Why do some websites have ?utf8=✓ in their title?

URIs contain utf8=✓ to force the client to send UTF-8.
It works because the key-value-pair (which is ignored by the target) contains a unicode-only character.

From Is the use of “utf8=✓” preferable to “utf8=true”?:

By default, older versions of IE (<=8) will submit form data in Latin-1 encoding if possible. By including a character that can’t be expressed in Latin-1, IE is forced to use UTF-8 encoding for its form submissions, which simplifies various backend processes, for example database persistence.

If the parameter was instead utf8=true then this wouldn’t trigger the UTF-8 encoding in these browsers.