grammar - Future Simple and Future Perfect

I've read the question about Future Simple and Future Perfect, but I still can't understand the exact usage of these two tenses.
Is there any diffrence between the following?

I will do my homework by 6 p.m.

I will have done my homework by 6 p.m.

I will do my homework after school
I will have done my homework after school

Also, I've checked the word 'by' in Oxford dictionary and there're some confusing examples:

By the time (that) this letter reaches you I will have left the country.

Can I use 'By the time (that) this letter reaches you I will leave the country' instead?

Answer

The difference is in the reference point of time where the speaker perceives him/herself to be.

I will do my homework by 6 p.m.

The time is sometime before 6pm, and the speaker is simply asserting an intention to do the homework before the stated time.

I will have done my homework by 6 p.m.

The time when this sentence is spoken is the same as before, but the speaker is moving the reference point to some time in the future after 6pm. Effectively s/he is saying that "When we next speak (at sometime after 6pm) to see what my homework status is, I will be able to say that I have done my homework".

The future perfect tense is useful when discussing something else that is to happen. For example

"Let's go to the cinema tonight. The movie starts at 7pm and it takes half an hour to get there. Can you come?"

"I think so. I will have done my homework by 6pm, which gives us half an hour to eat something before we need to leave. So yes, let's go."