Why would an artist release a single after the album containing that song has already been released?

for example: why did Eminem release his 4th single of Recovery, "space bound" almost a year after the album was released? or even his 3rd single of recovery, "no love" 4 months after the release of Recovery? This was common back in the day. Singles become hits before and after albums are released. Singles are used to not only make money, but to increase interest in the albums. So if people didn't buy Recovery , but like any of the singles relased after the album, they may buy the album. It's a marketing gimmick really.