为什么要使用pip install requirements.txt而不是pip install ./?

我对Python的依赖项管理不是很有经验，但是似乎可以将 requirements.txt 中的所有依赖项都放置在 setup.py 文件中在 install_requires 字段下，如下所示:

I'm not very experienced with dependency management in Python but it seems that all of the dependencies in requirements.txt could alternatively be placed in the setup.py file under the install_requires field as follows:

setup(
    ...
    install_requires=['numpy=1.2.3', 'pandas=1.2.3']
    ...
)

由于使用 setup.py 文件进行 pip install ./，与 pip install -r requirements.txt 相比，它还提供了许多其他功能.，后者有什么用?一个项目有没有同时使用两者的正当理由?

Since pip install ./ using a setup.py file also provides lots of additional functionality compared to pip install -r requirements.txt, what is the use for the latter? Would a project ever have a valid reason to use both?