URL Encode in php

I have a query regarding urlencode in php. So far what I found is that we use urlencode for converting a string variable into url format. But is there any example that if I want to us this on simple php website how and why I should use this? I searched a lot but I did not find any simple solution all are of almost of advance level and I am a learner of php. I want to learn this function from basic. May be so far I learnt wrong but please anyone would like to correct me if I am wrong. Can anyone would like to guide me?
Thanks in advance.