Student or Learner
when does that tense used?
It's used to talk about something that will be completed in the future. It hasn't happened yet. 'Perfect' indicates an idea of past tense so another way to call it is 'past in the future'
For example, I will do an exam next June, so right now I can say, 'By next July I will have done my exam.'
Did you look at the glossary ?:
Future Perfect - Glossary Definition - UsingEnglish.com
Notice with the added examples, that the word 'by' is frequently used because we want to indicate a time in the future.