This is essential. Research needs funding, and professors are working for grants in a manner deceptively similar to contract work. The grant work might not align well with what the community views as novel research so satisfying grant and research goals can be very challenging.
In the end I believe the most important focus of a PhD is to publish three papers at great venues in something that amount to a thesis topic. And don't distract yourself with something "fun" until this is done, because it is extremely common that the research topic will stop being fun at some point during your 5-7 year journey. People, including your advisor, will ask you to work on problems unrelated to your thesis when it fits their interests so it is up to you to manage your time. Since a PhD labour is cheap skilled labor you can not expect anyone else to take the high-road and manage your time.
In the end I believe the most important focus of a PhD is to publish three papers at great venues in something that amount to a thesis topic. And don't distract yourself with something "fun" until this is done, because it is extremely common that the research topic will stop being fun at some point during your 5-7 year journey. People, including your advisor, will ask you to work on problems unrelated to your thesis when it fits their interests so it is up to you to manage your time. Since a PhD labour is cheap skilled labor you can not expect anyone else to take the high-road and manage your time.