maybe i don't know how to communicate properly,
but whats stopping the user from,
-using Tv n adjust the shutter to get the aperture u want, or
-using Av n adjust the aperture to get the speed u want?
its just a matter of convenience n getting use to it,
in terms of image produced,
there shouldn't be any difference.
1/125 f8 is still 1/125 f8,
be it Tv, Av or manual(ok, not really for manual, but diff is sml).
Ah... if the setting is the same of course there will be no difference
But for a given photo, you must know what effect you want so as to know when to use Tv or Av
Take for example a flower as the subject with the grass behind as the background in a windy day.
If you want to focus in on the flower, but does not want the distraction of the background then you should use Av mode. This will make the flower sharp with blur background (bokeh effect).
But hold on! The wind is blowing and hence the photo will not be sharp cos of "shake". So in order to prevent "shake", need to just open the shutter for a very short time to freeze the movement. Hence the Tv mode is needed.
But if there is no wind, then of course shutter speed does not matter, if you have steady hands. The camera will automatically adjust for the exposure by change the shutter speed accordingly.
The was an expert who says that everytime someone ask about the shutter speed and aperture setting for his photo, he just give any number because by themselves it does not mean anything. The important thing is what effect you want the photo to be and then adjust the setting accordingly. It does not matter whether it is f/8 @ 1/1000s or f/11 @ 1/560s (say) as long as the photo gives you the effect you want