Without a valid Will, your estate passes under the rules of intestacy. These rules follow a fixed order of relatives and take no account of your wishes, your relationships, or the people who depend on you.
Unmarried partners and stepchildren can be left with nothing. Assets can be divided in ways that force the sale of a family home, and disputes between relatives become far more likely at the worst possible time.
A well-drafted Will keeps you in control: it names who benefits, appoints the people you trust to carry out your wishes, and lets the waterfall of your wealth flow exactly where you intend.