Notes

Labels generally have writers. These writers often have a singer in mind when they write their songs. Sometimes, a label has a writer write something for a specific singer, as in they create a team of writers for a singer for a given album.
When the label writers are not working on that stuff specifically, they basically are writing songs for the label to "bank" and occasionally singers are able to then peruse through them and choose which they like.
For instance, Miley Cyrus' "We Can't Stop" was originally written for Rhianna, but she decided that she didn't want it for her album and it was shopped around to other artists. Miley picked it up and then we all got to see her twerk...(got to see/were subjected to see whichever you prefer)
Not all of these songs are shopped around, but if the label is confident they can make it a radio hit, they will shop it around to artists or if they have an artist contractually obligated they will just tell them to sing it.
Source: I've written a couple songs for a few bands signed to some labels that you will likely never hear of...they paid me a pittance for it at the time and let me have a copy of the tracks as the bands recorded them. I got writing credit inside their album covers for the song - but that doesn't mean much since a lot of people download nowadays. I still write, but not so much for labels, just for fun.