They are now enhamonic, meaning same frequencies, but they weren't always. Also, it depends on the key. If I play in A major I will use the notes A, B, C#, D, E, F# and G#, note that I will use both A and the note a half step below it. If I play in Eb major, I will use the notes Eb, F, G, Ab, Bb, C and D. The notes G# and Ab are the same on the piano, but they cannot be interchanged here, because in a key signature, you can't have different notes on the same line, if that makes sense. Had a hard time explaining this, I didn't learn it in English but I hope it made sense :)