why I choose manual mode?
I prepared the answer xx years ago when I gave the lecture to my students.
As we know, EV(exposure value)=k*A*T where A ---- aperture, T ---- shutter speed, k ------- coefficient. In order to get a "correct" EV, the bigger aperture, the smaller shutter speed. In the normal case, the coefficient k is a constant. That is, if you reduce the aperture 2 stops you have to increase shutter speed 2 stops. For an automatic camera the exposure meter will give out the aperture value if you select a shutter speed (T mode, or shutter priority ), or converse, it's A mode (aperture priority). If you choose "P" mode you don't care about aperture and shutter speed, the meter gives out the exposure value, which is automatically controlled based on the formula EV=k*A*T.
But, but, but ............
in the cases such as too dark, too bright, the coefficient k is not a constant. Or we say the reverse law is void. In these cases, there is a bigger error if you use auto or program mode without a proper compensated EV. So in these cases choosing the manual mode is easier for you to get the "optimized exposure". As an experienced shotbugger, you should know why you increase the EV when you take at the snow scence, and reduce the EV at sunrise or sunset.
My two cents.
PS: Even 10D, there is no point metering, setting the metering mode as "paritial" is better.