This is a capability that they put into Project that from the "proper Project Management procedures" I can’t stand. If you are trying to be precise and accurate in your scheduling, then this option is very bad.
The only cases that I have seen this used "properly" is when a schedule is being built as a proposal and is still in EARLY draft stage. But, if somebody isn’t coming back and updating the fields with the right type of data, then it is useless. Also, it makes reporting interesting when you have a date field with "Talk to Bob" written in it.