There is a difference here;

In the FastAPI version, you need access to the app object (or a router) which you can register your routes on. This makes building larger / complex applications more difficult, as it requires the entity at the bottom of the hierarchy chain (the route) to know about objects higher in the chain (the app / router).

Litestar's approach encourages decoupling and modularisation. For example, it's very easy to re-use route handler objects:

@get("/")
async def some_handler() -> None:
 ...

v1_router = Router("/v1", [some_handler])
v2_router = Router("/v2", [some_handler])

It also enables a truly stateless and modular approach:

# app/some_module.py
@get("/")
async def some_handler() -> None:
    ...


def create_some_router(base_path: str, guards: list[Guard]) -> Router:
    return Router(base_path, [some_handler], guards=guards)
 

# app/app.py
def create_app(debug: bool) -> Litestar:
    some_guards = []
    if debug:
        some_guards.append(require_authenticated)
 
    return Litestar([create_some_router("/v1", guards=some_guards),])

I wouldn't say that you need to manually register things in Litestar, I'd say that Litestar allows you to be more flexible and explicit.