Ultimately, all of us will die. It might not be today or tomorrow, but we know it will happen one day. At that time, especially if you have children or dependents, you will want to know that they will be looked after when you are no longer around.
By having a Will, you are able to pass on your assets as you wish to the people you choose to receive them. If you do not have a Will, then your estate will pass in accordance with the intestacy rules and may be distributed in a way you would not want. If you want to ensure that the right people are able to inherit your money and possessions, then you need to make a Will. If you have a partner but are not married or in a civil partnership, then having a Will is even more important because they will not be eligible to inherit your assets under the intestacy rules so could end up with nothing.