It means if someone is looking for icons for multiple characters (say they just want to find some good season 5 doctor who icons in general and aren't picky about characters) they can use the two-tag search to narrow things down.
They could use "doctor who: new who" and "-season: 05" and boom. They'd get all the major season 5 doctor who icons on one page. One trip instead of having to check each character tag or scan the whole tag.
Some people will be using the site just to pick up general icons so that makes it so they can narrow down the fandom tags while the memories can be used for people zeroing in one character.
no subject
They could use "doctor who: new who" and "-season: 05" and boom. They'd get all the major season 5 doctor who icons on one page. One trip instead of having to check each character tag or scan the whole tag.
Some people will be using the site just to pick up general icons so that makes it so they can narrow down the fandom tags while the memories can be used for people zeroing in one character.