Tails is good because it's loaded into memory and doesn't persist after using it, unless you want it to. As soon as the machine turns off the memory is lost and no trace of what you were doing can be found. It also runs on Linux which out-of-the-box is miles better than Windows and other generic big brand operating systems, especially when it comes to security. There is a reason why Linux is the backbone of the internet and why it is used by professionals, academics, researchers. It's also why YOU use Linux when you use Tails - because the people who make Tails are security experts who know how superior Linux is when it comes to both defensive and offensive security.
Windows is off limits to the end user. You think you own the operating system but you lease it from Microsoft as part of a lengthy complicated legally binded contract which gives Microsoft the right to take back their product. You borrow Windows when you use it. With Linux the philosophy was always to give power back to the user and so nothing in Linux is above the user and everything is yours to do what you want with, whereas in Windows you are a low level consumer with the basic assumption that you are not the ultimate authority over the system. In Linux all you need to do is type a simple command into the terminal;
And you are now at the center of the entire operating system as the ultimate authority. Nobody is above you. You can essentially start deleting system files if you really wanted to or fundamentally change the OS down to the tiniest detail and people do this, specifically people who are Linux power users; sysadmins, computer scientists and programmers, web admins etc. That is also why it scares most people off because it's not a pretty affair. There are lots of commands, lots of staring at a terminal window and lots of knowledge about how the system works in order to do what you need to do. With Tails, all the geeky stuff is taken care of but the end product is a very refined and customized OS running on Linux that has all the benefits of what Linux stands for as well as being setup specifically with adding extra layers of protection.
It's not the ultimate solution though. You have Whonix, Kali Linux, ParrotOS, Qubes OS, Linux Kodachi etc.
And really when you talk about how good Tails is you are essentially talking about info security in general and what it takes to protect devices and networks. Your protection is only as good as the awareness of what you need to protect and how, when, why etc.