What do you mean why is it a thing nowadays?

Reproducible builds are an essential method in industrial computing - Debian isn’t at the forefront of this, it is merely adopting industry wide techniques also applied to other operating systems in use in long-term and safety-related applications.

Certainly, a lot of the hard work of the Yocto and Debian developers is already in your hands.

What is interesting is that this is now being applied in a more forward-focused policy by the Debian developers, that it will now be the norm rather than an option…