Question

What does it do? I read that it downloads things from Stdin, but where do you actually need it?

Conclusion

some_program | wget -i - -P ./directory

wget gets urls as Stdin from some_program. The input will result in output generated by wget to ./directory.

wget -i ./file

The above command gets urls form ./file, and it generates output to the current directory. The crux difference is the lack of – sign to the above command.

Answer 1

I can imagine it being useful when piping output from another program:

linkdiscoverer | wget -i - ./directory

(For some suitable linkdiscoverer program.)

I’m not sure the ./directory bit is really what you want though. Did you mean to use -P ./directory to save files into the specified directory?