They will help you in your listening and maybe pronunciation skills. If you are an advanced learner they may also introduce you to new vocabulary. :)

Yes they do, more practice is better, but it is not the most best. The best is living in America, Canada, or the UK for the summer or for a one year. Second best is working with a tutor or taking a English class.

yes

Yes, movies can help you learn English. Watch movies in English, but by using a simple subtitle in your native language :)

"I have an important question. Hi everyone, I want to ask whether movies can help me in developing my second language (English) or not. Bye bye." Yes, they can help you, but you need to pay attention and take notes. If you rely on subtitles too much, you may end up "reading" the movie instead of watching it.