If you don’t make a will and you die intestate the government have a laid down procedure for the distribution of your assets. That distribution may not reflect your wishes and will certainly not be tax efficient. Your estate may not go directly to spouse and/or children as you expect.
When someone dies it is the people left behind who have to pick up the pieces of sorting out and administering the estate. The administration is much easier and cheaper if there is an up to date valid will.
You can take the pressure off your loved ones by detailing your specific funeral arrangements (hymns, burial or cremation etc.) and even pre paying your funeral expenses.
You can give specific items which may be valuable or just sentimental to friends and gift money to favourite charities.
In modern times many couples live together unmarried or same sex partners. Neither partner has any legal right to the others estate without a will.
Step children need protection also. Only legally adopted children have the same rights as blood children.
In Scotland, marriage does not revoke a previous will, so as you can imagine it is extremely important to rewrite it after settling with a new partner.
You can appoint guardians to look after your children up to the age of 16 (18 in England) should something happen to you and your partner in an accident. You can also appoint trustees to look after the children’s money until they have reached a more mature and sensible age such as 21 or 25.