When you read, notice the idioms you see in use, check the meanings in a dictionary and then try to use them. There are thousands of them and far too many to remember by learning. We also have quizzes on them. ;-)
People do use them a lot. You can speak without them, but you may have problems understanding things if you don't know any. Anyway, we have given you a list so you can check if you read or hear one. It's up to you whether you learn them. ;-)
My english teacher, in Phoenix Arizona where I am now to sharpen my English, asked me to check out Using English.com site. As English student, I hope this site will help me to improve my English. Thank you for the opportunity.