why one would use "północy/południu" for North/South?
It's because of Polish grammar.
Did you ever heard about noun declination? In Polish language we have 7 different cases, which means it's possible for one noun to have 7 different endings in singular number and another 7 possible different endings in plural number.
północ/południe - means north/south
(where?) na
północy/południu Where do you live? I live in North/(or) South Poland . Na północy/ południu Polski.
some other endings:
( with) z
północą/południem