package management - Why should I regularly run 'apt-get update' and similar commands?

I am completely new to Ubuntu. A friend recommended me to frequently run the following commands:

sudo apt-get upgrade

sudo apt-get update
sudo apt-get autoremove
sudo apt-get autoclean

He said that this will somehow keep my system clean and updated. I do not know what exactly these commands do. Is it really useful to run them regularly?