Dear Ana Alvim,
"Why should we say "I sit in front of the computer" when there �s nothing behind?"
Similarly to Bruce and to others, I ask: "What difference does that make?"
I don �t understand your problem.
Here are some sentences similar to your sentence.
"Why should I say, �I have a coin in my left pocket �, when there is nothing in my right pocket?"
Pardon? What a strange comment to make!
"Why should I say, �I have a chair underneath my body �, when there is nothing above my body?"
This is also a strange comment to make!
"Why should I say, �There is a train coming into the station �, when there is nothing going out of the station?"
This is another strange comment to make!
"In front of" is a form of preposition, (which uses three words). It is similar to the preposition "before".
The opposite of "in front of" is "behind", (American English "in back of"), but it is not necessary to use this opposite word in the same sentence. There MAY be something behind the computer, or there may NOT be, but whether there is or not is totally irrelevant.
"I sit in front of the computer".
"I sit before the computer".
"I sit at the computer".
The three sentences convey the same message. Please, use whichever one suits you best.
I hope that you don �t think that I am attempting to insult you! I chose three stupid sentences just to try to show you how strange a question you had posed to us.
If I have misunderstood your query, I apologise.
All the Best.
Les