将 does not have to be added to express future sense: it depends on the context.  For example, in the sentence you provided, the tense has already been set:

明天我在北京出差

明天 already places the occurrence of the verb-object 出差 in the future, therefore 将, although not grammatically incorrect, is not necessary here.  将 is not needed when you have already set the present tense by using such "time words" as 明天，下个星期一，两天后 and so forth

将 is more useful and necessary in sentences that refer to the future, but no time word/phrase has been used to set the tense.  Take your example sentence again:

我在北京出差 - I'm away in Beijing on a business trip
我在北京将要出差 - Soon I will be going on a business trip to Beijing

Notice the addition of 要 : quite often when 将 is used to express the future in this way, it is couple with 要

Hope that clears it up - any questions, let me know