One key reason is that there are different rules that apply to profiles that are very recent, vs. those that are within the last 100-150 years, vs. those that are older than 1700, and also those that are older than 1500. By putting at least an estimated date in one of the fields, the system will understand how that profile is to be handled. Technically if you don't put a date in there, the system "could" imagine you meant born year zero and died year zero and lock it out for all except those who have a pre-1500 badge (I don't think it would "technically", but it's a possibility)