why is repeating things a good thing?

Repeating what has been said can remedy the lack of attention the listener may have had the first time(s) it was mentioned. Repetition might also suggest that this message is more important to you. (Also, if there is a time gap, the listener may have forgotten the message.)

It is more effective if the same idea is repeated in different words, just in case the listener did not understand the first version(s).

To make sure that the listener understands what has be communicated, ask them to summarize (give you feedback) about what was said in their own words. However, what they understand may not be the message that you sent. Asking them to give the message back to you will help you to judge their level of understanding.

Merely asking, "Do you understand?" is not adequate, because almost everyone will say, "Yes."

See <http://www.members.cox.net/dagershaw/lol/communicationDepths.html>.

I hope this helps. Thanks for asking.

Because if you repeat things at school you learn them by memory that is why.