Why does "grep '[^ -~]'" behave identically to "grep -P -L '[^[:ascii:]]'"?

Question

I've been assigned to grab files that only contains ASCII text. I found this code online but just don't get it.

grep '[^ -~]'  $someargument

I found this has the same functionality as:

grep -P -L -r '[^[:ascii:]]' $someargument