Lulu Pinney goes over the subtle art of working with significant digits:
When we say on the phone "I'll be there in half an hour" it’s quite likely we'll arrive sometime in the next 25 to 35 minutes. But for the context of meeting up with a friend "half an hour" will do. If you said "see you in 27 minutes" that would raise a laugh being an odd level of precision for the given context. The same ideas apply to numbers in journalism.
Important in both accurate representation of data and readability.