Unless you're very likely and have inherited the family's birthday book with all the birthdates of everyone who ever popped in to say hello, birthdates can be hard to find.
Birthdates are helpfully given in the indexes of deaths taking place here since 1969.
Why are they useful? Well, take my relative Jessie Smith. By finding her birthdate (from the London parish registers at Ancestry), I was able to pick out her death entry very easily, even though she had got married and changed her name in the meantime.
I've used a birthdate to help prove people are related, including Caroline Jones who lived to be over 100 - I was initially rather suspicious of this, but the family bible confirmed the birthdate given at her death.
Another source of birthdates was published last month, the names of minor children given when soldier's registered to fight (Enlisted) in World War One.
Among those was William Chappell of Penzance, whose daughters' birthdates are both given. Also Charles Chipperfield of London Docklands - in this case the daughters' birthdates helped me bypass the fact they married under subtly different names.