'04 P-bass question: Push/pot volume knob?

I've been looking around lately for a good p-bass and I just came across a 2004 MIA for a really good price. I'm no expert on P's and their features and this one has a volume knob with a button in the center that you can push in. When engaged it seems to cut the lows somewhat but certainly doesn't sound better to me, actually makes it hollower and empty sounding. What is the reason for this button? Why do I not see this on older and newer P's? I'm pretty sure sure this is a passive bass as I see no battery compartment. The bass sounds pretty good with the button unengaged, as least as much as I can tell in a crowded music store. If I buy this bass is it possible I might have regrets down the road for any reason and feel the need to change the pot, knob and or pickup?

I realize that question is very subjective so I guess what I'm asking is what are peoples experience with this setup? Why did Fender do it cause it sure adds nothing to the tone for me.

 

I A/B'ed it with a new Am Standard and found the output a little lower volume and not as bright sounding as the new one. This could just be the difference in new strings on the new bass. Regardless if you engage the button it sounds like you just threw a wet blanket on your amp. I could live with that because you don't have to engage the button but I'm curious as to what other's thought about any tonal or mechanical differences of this setup for better or worse. I'm not big on bells and whistles and when an "improvement" is introduced and then abandoned I become quite skeptical.

 

Thanks for the info. Now that I know what it's called I've done some searching and found the general consensus is exactly what I thought, it's as useless as teats on a bull!