Regarding withdrawal of a paper which has not been registered for conference I had submitted a paper for an IEEE conference. My paper has been accepted for presentation and potential publication. However, I have not paid the registration fees and do not wish to  register the paper. I want to submit the same paper to some journal. So, is it necessary for me to withdraw the paper from the conference by writing some sort of email or Can I simply submit it to some other journal without withdrawing it from conference? I mean what is the need to withdraw the paper if I have not even registered it. Non-registration would automatically mean withdrawal
Some conferences will publish even without registration / payment (exceptions, special support, considered worthy etc), so you have to contact them to properly withdraw the paper.

You must properly withdraw the paper before you submit it to another journal as you cannot submit the same paper twice.