By writing a will you, can precisely determine who will inherit your estate and let loved ones know you have considered their needs. Without a valid Will, your property will be distributed by law and not necessarily how you would have wished.
Writing a Will takes away added stress from the family after your death. You can choose who will benefit from your estate and who will manage your affairs after your death as executors.
If you have children under 18, you can nominate (subject to family court approval ) a guardian to decide who they live with, which schools they should go to and generally manage their wellbeing if they are left without a surviving parent or other person with parental responsibility.
If you have a preference for a burial or cremation, this can be included.
If you have substantial assets a Will gives the opportunity of reducing your Inheritance Tax liability.
In summary, it is probably true to say that every adult, who has the mental capacity to do so, should have a Will in place, and regularly review it once they have one.
If you are married or in a civil partnership, both you and your spouse/civil partner should make Wills either to ensure your spouse is the main beneficiary, and/to make sure your respective children from previous marriages/relationships receive the share of your estate you wish them to (and your spouse/civil partner avoids opprobrium, and potentially expensive legal dispute, from those children once you have gone
If you are living with a partner and not married, it is often even more important that you have Wills and name each other as the main beneficiaries as if you die without a Will ‘intestate’ the rules of intestacy will provide your partner with nothing from your estate (except anything passing outside your Will/intestacy through your joint ownership).