A will enables you to control how your estate is administered and distributed after you die.  Your estate may include your house and any other property that you own, your personal possessions, money in bank or other accounts, investments, savings of any sort and any business assets or shares belonging to you personally.  It may also include your interest in assets that you co-own with other people.
A will allows you to decide who should inherit, how much they inherit, when the inherit and who is responsible for collecting and distributing your assets.  It also enables you to make gifts to charities should you wish to do so.
If you have children, a will enables you to appoint guardians if both parents die whilst the children are under 18.  If you have children, it is vital to provide as much security and certainty as possible both for your children and for the wider family with responsibility for them.
A will also enables you to express your wishes over what type of funeral you want.
A properly drafted will can prevent (or put to rest) family disputes which could otherwise arise.