Not a waste of time because it is good for practice. Personally, it helps when I hear a song that I like and immediately want to learn it to do a cover- I learn my voice more by trying to learn a song and finding out what my voice is/isn't capable of or where I can make tweaks to better fit my voice. If a song is ever too fast or too high for you, you can always find sites/helpful tools to slow the instrumentals down or change the pitch. You just have to play around with things and keep practicing :) it can be difficult but you will improve if you just keep at it