Just a quick note - to summarize:
If you want to use all allocated space on tape media you can use "Do not create, continue using the current media set." However, with this option on it is hard to identify on which tape what backup files resides as it could be spread all around different tape media, so when it comes to restore operations you may need more than one tape.
The option you are currently using – “Create a new media set for every backup session.” is much more granular. You will have one backup session per tape and in this case it is much easier to identify what tape media should be loaded into the drive to run the restore operation, but it does not use all allocated space.