You asked the right question. They do it because some people like running more than 100% and then driving that into a linear amp. This is why some Hams AND CB operators get upset. The net result is if a station transmits with gear setup in this way; they will 'bleed over' many channels above and below the channel they are actually on. Not to mention, the second and third harmonics of that fundamental transmit frequency (~27mhz) can also be splattering on 54mhz (which is the 6m Ham band - BTW).
The worst part is trying to convince them that they are really just wasting power. Power meters are 'dumb'; they just read ALL of the power being put to it. It cannot tell if the frequency is 26mhz or even 28mhz. So, the person doing this is 'convinced' that they are putting out 500 watts on a given frequency. In reality, only a smaller portion is getting transmitted where they want it. They waste power and mess up frequency usage - the real net effect.
On the other hand, a Ham radio has very tight filtering and any adjacent frequency is unmolested when transmitting. When they are transmitting 100 watts of clean power on that particular frequency - you can be relatively certain that all of the power is going where it was intended.
The BEST way to run a CB radio is to be sure that it isn't 'bleeding over'. Keeping the modulation below 100% is one way. The other is to drive it with a power supply that doesn't budge from 13.8v when transmitting. This helps a little to keep internal distortion down; as running everything @ 15v can make for more IMD. Most importantly, is to NOT run a radio with a clipped out AM modulation limiter. That is a SURE way to make it splatter - and badly at that! Another is to run a low pass filter between the radio and a amp - or out to the antenna if no amp is used (this doesn't always make things right with your neighbors - BTW). If using an amp; make sure the voltage is constant when transmitting and use an amp that is Class AB biased. This will keep the amp running more efficiently and create less 'bleed over' as well. Classs C biased amps are notorious for wreaking havoc by splattering everywhere. Avoid them!
Not quite sure that I have all of my 'ducks in a row' over a couple of my statements. But I'm sure that someone will come along and correct the finer points I made.
If D11 has one lead clipped; then just solder it back together with a low-watt soldering iron. This will make VR4 active once again. Just make sure it isn't up all of the way; or you will have to turn it down.
