Why do artist still release singles? Especially now that with streaming services, you can buy/listen to individual songs.

Especially when a single releases after the release of an album. 
In the past, artists would go a long time between releasing albums, disappear, and then come back as a big event. However, in recent years, there has been a shift toward artists releasing more music and maintaining a presence in the conversation. [2] Artists, especially younger acts trying to establish a commercial foothold, are releasing more pre-album singles than ever before. [1] This approach reflects the breakneck pace of a world driven by streaming and social media. [2] Since streaming gives people so much access to music, simply keeping an artist's name in people's minds can be difficult. For a fledgling indie artist who's just beginning their career, putting out a single shortly after an album is a way to maintain a presence in the conversation. [3] More generally, artists tour more often to make a living. As a result, they need to put out more music to make money. [4]
Artist still release singles despite the fact that fans can already listen to individual songs on streaming services, because doing so helps them stay relevant and build a loyal fan base faster. [1, 2] For example, by releasing singles, artists and labels can "keep things flowing" and hopefully avoid one-hit wonder-dom, which appears increasingly common in the era of the viral flash-in-the-pan. [1] Similarly, keeping multiple singles in the market at once allows labels to vet a variety of tracks and gauge listener response before going to radio. [2] Moreover, by releasing singles, artists stay relevant in a music market where releasing music only 1 or 2 times a year is almost the same as releasing nothing at all. [3]