This answer will apply to all scale types. For the sake of example, though, let's say you were playing a melody within an E minor pentatonic scale. If you only knew one postion, let's say the one that starts on the 12th fret of your low E string, then you'd only be able to play a couple of octaves worth of pitches across all 6 of your strings. What if you wanted to play a melody several octaves down that your current position could not reach? You would need to know what scale shapes are available to you when you use frets below the 12th. In short, each scale position only covers a couple of octaves worth of pitches, so if you want to be able to play in the lower registers and the higher registers of your guitar for each scale, then you would need to know all 5 scale shapes available to you.
I hope this helps! Let me know if you have any questions.