Because every misprint encodes (i.e. writes) a wrong movement into your muscle memory. An accidental misprint does not matter much—it will be rewritten by correct presses; but if you consistently make the same misprint, it is not only encoded—it is consolidated, that is, a long-term motor program is created for this wrong movement. And then you will have hard time trying to get rid of it.

If you focus on speed rather than on accuracy (when learning), you encode wrong movements into your muscle memory quickly, which, of course, is a sort of an accomplishment too.

No, because the structure of a modern touch typing course makes it unnecessary.

Moreover, if you cover your hands, this can hurt you because you (and your teacher if any) cannot check visually if your hands maintain the correct, safe posture. This is especially important in the beginning of your learning, because you are not used yet to the correct posture and have to control it consciously (and visually from time to time).

Details

The advice to cover hands is sometimes given to force a beginner touch typist to type without using the sense of sight to find the keys. But this is dangerous because prevents checking the hands posture.

The structure of a modern touch typing course makes covering hands unnecessary. The idea behind the structure is the following:

The first exercise is elementary. Anybody can do it without looking at the keyboard.

The second exercise is a bit more complicated. The increase in difficulty is so tiny that it also can be (relatively) easily typed without looking at the keyboard.

The same with the third, fourth, and so on exercises. Every exercise is only slightly more complicated than the previous.

When you follow this very gradual learning curve, the desire to look at the keyboard almost never appears.

How to make this smooth learning curve?

This is where computer can help. A special computer program can create and arrange exercises to ensure that every exercise is only a tiny bit more complicated than the previous.

So, to summarize:

Work with a modern course and never cover your hands.
Control the posture of your hands to save them from injuries.