A buffer allows the inherent voice of your pickup to travel unencumbered through the resistance of pots and cables and reach the amplifer unencumbered and uncolored. The result is twofold:
1. Your guitar's pickup will sound clearer, more articulate, and with broader frequency
2. Turning the volume control on your guitar will not also affect the high frequency tonality
If you want to hear the true full spectrum sound of your pickup, you need a buffer. If you want to use the volume control on your guitar without changing the tone, you need a buffer.