The world wide web is a vast space. There are so many people using it. Some of them have websites. These websites often differ from others. Said people usually put a great effort into creating these websites with a great design. Some also cared enough to add a dark theme to it.
And then they hide it.
Or do they hide the light theme? It doesn't matter. The point is that they don't add a theme toggle or a way to switch between dark and light modes to their site. They only check which color scheme the user prefers and call it a day.
This results in a worse user experience for people like me. We’d have to go to the system settings or browser settings or whatever and change our preferences. Only to find out that there is only one theme, and we wasted our time. But let’s ignore that.
For a smaller niche of users, it gets even worse. I use Librewolf as my browser. Per default, and for a good reason, it has RFP (Resist Fingerprinting) enabled. Librewolf makes it impossible to change the browser theme without disabling RFP first.
I don't want to fiddle around in my settings. I don't want to disable RFP. All I want is to see the web in all its colors.
It might only affect me, might many others. But adding a way to switch between your website themes is almost no effort and is always great to see.