I know there are several ways of using the Future Tense, one of them being "be going to". This is used for intention, something which has already been planned, for example:
We're going to visit my sister this summer.
Will is used for deciding something at the moment of speaking, eg. "I'll make some coffee." However, why do we use the future with will when we say "I'll be in London next week" or "I'll be in the office tomorrow" when these seem things which have already been planned and are intentional, so why not use "be going to"?