They are used by different people, with different restrictions. YouTube is more visual, hence more for those who don't use phones and tablets much, but are used to stream on computers ; music can be listened to while reading about it or finding lyrics, etc. Those who are used to watch dramas on the YT channels of some providers are also accustomed to look for music there too. " As of September 2012, countries with standing national bans on YouTube include China, Iran, and Turkmenistan." YT has been made more difficult to use ad-free, but that's still possible (half the users do not pay a subscription). Some other countries are making YouTube difficult to access in part, for contents reasons, which was one reason it got banned in China from 2008 (But it is still possible to access in China, with some technical tools, like a good vpn.)
Spotify is a digital music, podcast, and video service that gives listeners access to millions of songs and other content from creators all over the world. It targets younger people on the go, listening with earpods from their mobile device. It means another subscription and for users after one month trial, it is not ad free except paying one more subscription. As of nov.2023, acording to Google, "Spotify has approximately 365 million monthly active users in over 180 countries in the world. Despite the extensive global coverage, there are some countries where you can't access Spotify, including China, Ethiopia, Libya, Somalia, Sudan, Iran, Iraq, Myanmar, Cuba and Venezuela.". "In terms of technical quality, Spotify offers better sound quality than YouTube Music. In their free version, YouTube Music's highest quality of streaming is 128kbp, and Spotify's is 160kbps (it maximums at 128kbps for YouTube Music's web player)"
It may be good to have that bgm on both platforms for the different users, although indeed Spotify would be better for quality listening.