Your tone loses it's high end after going through long lengths of cable. Putting a buffer at the front will drive the signal and will reduce the loss.
Not everybody loves high end though. In the past Brian May and Hendrix used to use those long curly cables with the intention of killing some of the high end frequencies.
Though gear has been evolving and maybe at that time there were constant issues with feedback, tubes and piercing high end frequencies. Now everything is much cleaner and stable, and although you can take out frequencies with eq, you cannot add them back, so a driver is really useful in keeping everything there.
Check out the zvex super hard on demo clip where he does the unity gain thing. You'll hear an increase in 'clarity', 'presence' or 'sparkle'. Similar to what the SCP does. Without them, your sound will be slightly duller.
Very useful pedals as drivers, unfortunately they are very expensive over here. I understand the technology behind them and am experimenting on making one myself, which hopefully will be priced under $200.
