A will is a legal document that dictates the distribution of assets after your death. In Malaysia, wills are governed by the Wills Act 1959.
If you die without a will, your estate will have to be distributed in accordance with the Distribution Act 1958.
You definitely need a will if you are married, having kids, or having a lot of assets (e.g. money, properties, stocks and shares). You get to decide who will get your assets when you died, and you can appoint a guardian to look after your children. You can also choose your funeral arrangements. Having a will minimises the possibility of dispute arising amongst your family members.