It's useful for those who are interested in using an immutable OS. An immutable OS has several advantages over regular OSes, such as reliable and atomic updates, easy rollbacks, increased stability etc. However, a major drawback with immutable OSes is customisation - because certain key parts of the system are read-only, you may be limited in what you can change. Fedora Silverblue/Kinoite solves this to some extent by using ostree, and rpm-ostree to install packages in a layered fashion. The problem with that though is that it can significantly increase update times and also bloats up your system, which is a concern if you've got a lot of packages installed in that manner. Plus, there's also the issue of limited customisations - maybe you want to switch to a completely different DE for instance.

This is where making your own custom image comes in handy. When you fork the template, you can put in all your customisations on git, and your custom image gets built automatically.

You have to trust another party blindly not to inject malicious code

But you're doing that anyway with pretty much every piece of code/app/website that you use without auditing it yourself, so what makes this any different?

In any case, at least with uBlue you don't have to blindly trust anyone because everything in uBlue is fully open and transparent, you can see for yourself exactly what's being done - in plain text.

Risk of not being maintained anymore.

Not really an issue. This isn't a distro, it's literally just a script which builds your own distro using upstream code, which is pulled from Fedora. So the only real risk is if Fedora decides to shut shop or something. But even if they did, someone else would fork it, and you could just rebase to that fork and move on.

It's useful when you need something custom. Bazzite, an alternative OS for the Steam Deck and other handheld PCs, is built that way.

Or if you want one for your company.

For whom is it useful to write his custom image? Maintenance? Added value?

It's not good to layer too many packages on top of the normal atomic Fedora.

If you still need a special configuration, like for Nvidia ("silverblue-main-nvidia"), gaming setup ("bazzite"), non-supported DEs/ TWMs (e.g. "deepin-main") or special hardware (e.g. the Asus ROG or MS Surface), you can just download it and have everything in one go and working as a whole.

This is not only very convenient, but also more reliable, since your baked-in Nvidia driver for example/ config is the same as the one from thousands of other users, including the developers.
It prevents the "But it works on my PC"-problem and helps troubleshoot issues way faster and more precise.

Anecdotally, KDE has been a mess for me everytime after a few weeks for example.
This is due to package drift.
On my Kinoite install, it is actually usable and doesn't have nearly as many bugs as on the normal Fedora KDE variant. Just, because the install is the same.

When is it useful to deviate from fedora and use a ublue version? You don't have a fedora signed distro anymore. You have to trust another party blindly not to inject malicious code. Risk of not being maintained anymore.

Pretty much always imo.
Fedora isn't allowed to ship some stuff ootb, and uBlue tweaks it for you.
Also, you get more choice, as mentioned above.
uBlue isn't much derivated from the OG Fedora Atomic, and it has a large user base.

It doesn't need much maintenance, since it is mostly automated. That's the main pro point compared to forked distros like Nobara. Also, it's signed.