system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do radio stations decide what songs to play?

Does calling in for requests actually do anything? What's to prevent DJ's from playing two hours of the same artist?
gpt
I work in radio in Canada, and this is how we go about getting music on the air. 

Programming a radio station is a rather intricate process. There are a lot of factors that go into what songs are played on air. Most radio stations will do music surveys and ask their listeners to vote on what music they would like to hear or do some sort of market research. This doesn't guarantee those songs will be added but it gives the station a good idea of who is listening and what they want to hear. All radio stations also have access to BDS a tracking site that shows where songs are charting, what stations are playing what songs and how often they are spinning. Most stations have "mentor stations" (stations with similar format) that they monitor to see what they are playing, how often etc.  Once a Music Director decides to add a song it goes into a music scheduler program, we use Music Master.

With a music scheduling program, all songs are categorized based on the format of the station. For example, High Rotation, Medium Rotation, Songs to play only overnights or on weekends etc. These categories can be whatever the Music Director likes. Then rules are set in place. This helps with the "flow" of the station. The rules dictate how many of each category are played and in what order. In an hour play 3 songs from "A Category", 2 from "B Category", 1 from "C Category" and every other hour play 1 from "D Category" etc. Other really important things to work out are things like artist separation (how often you hear the same artist) and song separation (how often you hear the same song). In a top 40 station, this is VERY important as the stations playlist is generally pretty small. For example. Taylor Swift can play once every hour, but her song Style can only be played once every 2 hours. Another example of a rule could be that you don't play two slow songs back to back.

From there the magical scheduler program compiles a playlist or "log" and the music director then goes over it to make sure no rules are broken. They can manually tweak it too. 

Seeing as how it's such an intricate process, a DJ who doesn't follow the log, or decides to keep adding songs to play what they want, most likely will be fired as they are breaking program rules that have been carefully selected. However, requests are legit (assuming you call with requests that fit the stations format ie. you aren't asking for AC/DC on a Top 40 station). If a station takes requests, most decide to do request hours. Which basically means they have created an hour of programmed music, but there are spots where the DJ can insert tracks or replace tracks if they get a request. The only challenge is that the DJ still has to abide by the rules in place (ie. can't play two songs back to back. Have to watch artist and song separation etc.).

That is a rather simplified version of how programming of a station works. It's very complicated. Lots of rules/formulas go into programming.

TL;DR - Rules are used to program a station, but requests do work.