Debian is very well suited for application servers, web servers, file servers, hypervisors (personally, we use Proxmox for some hypervisors and this is based on Debian), and much more. It can run on various CPU architectures and scales well from large to small. We have Debian DNS servers running on Raspberry Pis, and large application and database servers running on powerful Debian installs in our racks.
If you want an OS that is rock solid, easy to manage, and predictable, Debian might be the OS you're looking for. The official packages are generally far from being bleeding edge versions, but this means that you are using packages that are battle tested and will not break compatibility at any point during its life-cycle. This, to me, is one of Debian's core strengths and a good reason for recommending it to others.
On the flip-side, the lack of up-to-date versions of various packages may be an annoyance if you actually do need to be using the latest version of such a package. If you need to be using more modern versions, you will need to remedy that yourself or look at an alternative distribution. Ubuntu may be worth a look, in this case, being that it is basically Debian with newer packages and some minor differences here and there.
Personally, I would not recommend Debian for a graphical desktop OS. You'll nearly always be better off with one of the Ubuntu flavors for a modern desktop with all the bells and whistles you'd expect with a GUI desktop.
Kali Linux is especially well suited in environments where high security is needed for your custom developed applications. This is well suited for offensive testing your high security in house developed applications. It is also very good for doing penetration testing on in house developed applications and safe guarding them from external attackers. Kali Linux is tough for use as a day to day Linux operating system as this was never intended to be used by beginners and occasional Linux users.
Debian is one of the most well-thought-out, logical, and intuitive Linux operating systems you can use. Configuration, package management, file system layouts, etc, are all based on many years of usage, and it's very rare to find anything surprising or confusing in everyday usage. Package management and updates are handled by APT in such a way that once configured, you can almost guarantee your system will behave as expected for the lifetime of its service. Top-notch system integration provides all the tools you'd expect from a modern Linux system, with all the stability you demand from a demanding server OS.
I would say the number one competitor to Kali is Parrot OS. While they both do the same/similar things, both have over 600 pre-loaded tools. Parrot does use some more offer what may be considered a more intuitive user interface. Parrot OS has been around for longer than people think and still has active development a support. But Kali is used more broadly in Training sessions, demos and is considered to be form all the articles I have read, the gold standard for penetration testing and tool sets
Initial stages of figuring out back doors in an application hence making them secure.
ROI is good as it reduces in production security threats by letting us know well in advance.
Entire development team along with its designers get a huge sigh of relief as all security holes are sealed well in advance before going into production.
