If left on it will download any data that you have scheduled it to do so.
As for leaving turned on 24/7 there have been many conversations in the forum regarding this and the answer is pretty well divided 50/50. Some of us do and the others don't.
I think though that most of us agree that neither will harm your PC and the one thing we all agree on, even those that turn off at the end of the day, is that it may cause harm if you are constantly turning on/off during the day.
Generally if you are going to be using the PC during the day then leave on rather than turning off several times but when it comes to the end of your working day it's up to you to decide if to leave on, place in standby or turn off.