...because there might be some family members that you would otherwise not know about. This could be particularly important if you were looking for descendants, and especially in the case of families who had immigrated to England, about whom info prior to 1911 was sparse.
I am actually toying with the possibility of doing this. I would like to know if the only apparently-surviving child of a widowed father in a family was still living, and with the family, in 1911, as the father was interned in WW1, and disappears; so far, I have no clue what happened to the child. Problem is that so far I have 3 different relevant addresses for this family: (1) from the 1910 PO Directory (anyone happen to know specifically when it was compiled, or what info it was based on?), (2) from a marriage 6 March 1910 (but addresses given at marriage are notoriously unreliable, and this one is a few miles from any other family addresses), and (3) from a death 3 Oct.1911. Now, which one do I pick??? The death one seems the most stable, as I can match that address with the father's occupation - he was a baker, and this particular address had been used as a bakery for several previous censuses. But I need to know if he moved there upon remarriage, or was it a year or so later?