You won't be able to transfer music from your computer to your Fitbit device if you're the owner of one of the following devices. If you want to download music to your device, you will only be able to do so with a paid subscription to either Deezer or Pandora.
You can use the companion app for Mac and Windows computers to sync your fitness and music data. The software has been phased out in favor of the mobile app as it no longer recommends to download it on the setup page.
There is no way to download music to your device without a paid service. You may be eligible for a 90-day trial of either service if you sign up on the support page.
This change doesn't affect newer models, as they don't have the option to transfer files between your PC and Fitbit device If you own an older Fitbit that has the ability to store music, you will still get access to the music files you have on your device right now, you just won't be able to download any additional songs beyond October.
Many users use their Fitbit devices to control the music they play on their phone, whether it is on their phone or on their device. Having access to local music files can be useful if you want to listen to music offline or not have a phone.
It could be that there isn't enough demand to keep the service up and running. There was no response from the parent company of Fitbit.