I thought one reason artists released singles was to tease out an upcoming album release by getting songs popular on the radio and getting fans excited about purchasing the full album once released. In the age of music streaming and the trend of dropping entire albums at once (sometimes with little to no notice), what is the purpose of a single? In the case of an album releasing all at once, how does a song then become a single—does the artist decide, or is this based on streaming numbers, billboard charts, or some other factor?
Example: Taylor Swift released her concept album "Midnights" all at once, but now I'm seeing her song "Anti-hero" referred to as the single. When she surprised fans with the release of the "Folklore" album, "Cardigan" became the single.