
"Google employs synonyms automatically, so that it finds pages that mention, for example, childcare for the query [ child care ] (with a space), or California history for the query [ ca history ]. But sometimes Google helps out a little too much and gives you a synonym when you don't really want it. By putting double quotes around a single word, you are telling Google to match that word precisely as you typed it," informs the Web Search help center.
The plus operator was easier to use for single words and it was intuitive, considering that the minus operator is employed to exclude results that contain a certain word. Maybe Google wants to use the operator to integrate Web Search with Google+.

{ Thanks, 7E. }