It is always good to have a birthdate defined for many reasons. When a real or approximate birthdate is not known...
Since there are many for whom an actual or estimated birthdate can't be found, a speculative birthdate is a good alternative to help with all of the items in the link above.
For example, if a marriage date is known to be 1750, but you can't find birthdates for either person, you may speculate a wife's birthdate to be 1722/1732, and a husband's to be 1717/1727. This is based on averages over the entire website showing a husband's average age at marriage as 28, and a wife's average age as 23. Likewise if a youngest child is born about 1750, as this is often soon after marriage.
Is Date Speculative?: When entering dates, a checkbox is available for indicating that a date is speculative. Check this when a date is purely speculative -- then the date displays differently to make it very clear that it's a speculative date. Note that this is only intended for when the date is purely based on a guess, and not an approximate date that is based on actual sources. Like the speculation above, or in English, it's like saying "I know John was probably born around 1800 as his parents were born in the 1770's"
Then when you return to the person, it's immediately apparent to you that the date is a guess, and when visitors browse, they are alerted that your guess is not a date based on sources.
If a date is left blank for living people, go ahead and fill in the date but check the "Living-Private" checkbox -- then this person will not show for anyone but you, and this hole in the data is filled in.