The higher the resistive value the more high frequencies will get through, the lower resistive values will attenuate the high frequencies. A general rule is 250k for single coils and 500k for humbuckers. The 25k pot is used to keep the resistance low for the low impedance output of the buffer to eliminate the filter created by the interaction with the cables capacitance. Using a 25k volume pot on your pickups without a buffer will alter your pickups high frequencies in an adverse way.
Disclaimer: I make and sell Buffers. I also sometimes Modify, Build and sell preamps and amplifiers. My opinions are not intended to be sales pitches. I am a one man shop and prefer to spend time with my wife and family, i work full time on call and love to spend my spare time doing other things. I only make, modify or build things for those that seek them.