Wide aperture lenses (e.g; f1.4, f1.2) are difficult to manually focus in a DSLR which has only a plain viewfinder screen without a high-accuracy "split-prism" centre. A DSLR's normal focusing screen is generally used to frame a picture, but the camera handles the focusing automatically, so critical focusing by looking at that viewfinder screen isn't usually done.
When you DO want to focus manually, it can be difficult - though you wouldn't expect that! - with a DSLR's normal, bright focusing screen.
The Canon 'EG-S'-type "super precision matte" screen - which is easy to swap in and out instead of the standard bright screen - allows more precise manual focusing with, for example, an f1.2 aperture. HOWEVER, this precision focusing screen (it just clips into the camera's lens throat instead of the standard screen) is noticeably duller, or darker, than the normal screen.
So use it for wide-aperture manual lenses, but then it's best to take it out again and swap back in the normal screen if using AUTO-focus lenses.
Easy to clip in and out, makes manual focusing easier, but makes framing with AUTO-focus lenses dimmer or darker.