BMW recommanded that we should change the engine oil if we dont use the car after 25 days. why?

Could be 2 reasons I can think of, most likely you have an older model BMW that approved conventional oil for use, therefor they are worried about conventional separation in the pan from no use. Or on the other hand, yours is so new it has the new full synthetic 0W-5W oil for your 4-cyl, and it could become heavy when congealed with sludge from being idle so long. 0W-5W MUST remain light and viscous for low temperature lubrication, you don't want your motor blowing/getting damaged from startup just because of the oil weight do you? (Rhetorical)