Why would one use WebAssembly in the Backend?

Please correct me if I'm wrong, but WebAssembly is needed on the frontend because browsers only interpret/compile HTML, CSS, and JavaScript, so using any other Language would require a special tool.

Correct.

But the backend doesn't have this restriction;

Incorrect.

If a library or tool has a WebAssembly it does not necessarily mean it can be consumed by the JavaScript runtime (Node.js, Deno, Bun, etc.). Not by default. See How can I use a C++ library from node.js? for an example how one would bridge C++ and Node.js.

The runtime can consume other tools, that is not a built-in capability. In order to use another language, the library/tool has to expose bindings for the runtime. These are usually supplied by the tool maintainer. You can do it yourself, if you are developing a backend JavaScript application. However, you might or might not have access to its code; you might or might not have enough information to provide the bindings for your runtime to use. While a WebAssembly distribution is readily available.

Another problem that compounds using tools written in another language: different runtimes will require different bindings. Even if a tool exposes an interface for Node.js, it will not be usable in Bun. And vice versa.

Since WebAssembly distributions will work across all runtime environments, it makes them more portable. Admittedly, this is probably a slim use-case but it is still an advantage over bridging over to native code.

The portability also means it is easier to ship and deploy the backend application. Depending on the deployment environment, supplying a native module might be hard or straight up impossible, if the system is locked down enough.

There is also an element of extra security. WebAssembly code runs in isolated and restricted mode. This can be important consideration - native code can be a vector of attack.

Overall, there is no one single reason to choose WebAssembly. There are plenty of cases you might choose to access native functionality through a bridge. Or you might not have this choice at all and WebAssembly might be your only option. Or you might have a choice and lean towards WebAssembly not because of one reason but combination of several factor.

Tech is, at the end of the day, a toolbox. Not everything requires a hammer - you pull the right tool for the job. Some times a hacksaw would be more appropriate, another time might call for hitting things with a screwdriver.