It can lead to practical problems for those you leave behind.
The Administration of Estates Act 1925 (as amended) will decide who gets your property and possessions, and that may lead to costly legal disputes between relatives and alleged relatives.
It can mean that your assets are divided up and dealt with in ways you never intended, possibly resulting in hardship for your close family and benefiting those you may have not seen for years.
Do not assume that your spouse will inherit everything as a matter of course.
Any promises you make to members of your family or friends are likely to be worthless unless they are included in your Will.
Your personal representatives are also chosen by fixed rules and the only powers they have are given by Acts of Parliament passed years ago which are no longer adequate.