Many bands, and particularly rock and metal bands, play with their instruments tuned down a half-step. By pitch shifting the recorded music, you can avoid retuning your guitar or playing in a different key than the music is normally played on the recorded instrument.
Shifting the pitch to transpose the music doesn't change the speed of the music. You can separately slow down or speed up the music though.
Pitch shifting music is useful for more than practicing music recorded in a different key though, particularly for vocalists. Unlike other musical instruments, it's not possible to "retune" a voice. Shifting the pitch of the music is a good way for vocalists to practice singing songs that would otherwise be out of their normal range.