Why does msdb.dbo.backupset have entries for each database every 15 minutes?
Backups for SQL Server can be scheduled in different way and we do not know how it is set up in your case.That is why we cannot tell you where to look. Most common scenarios are:


SQL Server Agent Job
Maintenance plan
3rd party tool


If you run the script mentioned by @Kin in the comment section, it will show that your databases are most likely taking transaction log backup every 15 minutes.

If that is the case you also might want to purge your backup history periodically. You have to be careful on the first run as it might take very long depending on the amount of rows to be deleted and transaction log for MSDB can grow significantly. See the comments by Erik and Randi.