Backing up personal data, mostly stuff from my childhood that is irreplacable. Sure, I could just put them on a HDD, but then I’d have to replace it every 5-10 years. Data stored on Blu-ray can last a long time.
Data on hard drives also generally lasts a long time. Much longer than 5-10 years.
And make sure you’re constantly monitoring those discs, disc rot is very much a thing for all optical medica.
Assuming the drive spins back up after being left in a cupboard for 15 years, if you’re still even able to find a computer compatible with whatever cables it used back then. But yeah.
MDiscs are ISO rated for hundreds to thousands of years.
https://www.ecma-international.org/wp-content/uploads/ECMA-379_3rd_edition_june_2010.pdf