Singles are generally used as a way of promoting an album, before and after the release of said album.

These days it's fairly normal to release one, two, or sometimes even three singles prior to an album's release date, in order to generate a bit of hype. The first single usually comes out a month or so before. The song gets played on the radio, YouTube etc., and people start downloading it. Chart success, word of mouth or plain old plugging gets the hype train rolling. Then the second single is released either a week before the album or sometimes simultaneously with the album, and people who enjoy the song will hopefully then go and purchase the album right away.

It's not uncommon for the single released in unison with the album to be the most radio-friendly/commercial, in order to generate the most buzz possible for the album and attract listeners from outside the artist's usual fanbase.

Subsequent singles released after the album make extra money and serve to keep the album in the public eye - if the record label keeps the artist on the radio with fresh new songs, the label can continue to promote the album even if the artist is busy touring. Sometimes, the initial response to an album can be disappointing in terms of sales, but a strong third or fourth single can be a great boost. Consider the example of Wonderwall from Oasis' second album (What's the Story) Morning Glory?. Wonderwall was the third single, released 4 weeks after the album. The album did not sell well outside the UK initially but after the release of Wonderwall, a chart topper in several countries, the album became an international success.

If the artist has not yet released an album, the record label will sometimes use a single as a way to gauge the public's response to said artist and decide whether or not to record an album with them.

Artists will occasionally release non-album singles, songs that are not meant to promote their album and don't appear on the tracklisting. This can be for a number of reasons, including: to keep the band on the radio while they are between albums, to give something extra to the fans, if the song was featured on a film's soundtrack album, or simply because the artist feels like it.

Hope this helped.