Your personal reasons will depend on your individual situation, but some common reasons to have a will are:
Decide how your assets will be distributed - equal amounts to all your kids, donated to charity, etc
Avoid lengthy probate process for your heirs. Probate is the mandatory court process for dealing with a deceased person's estate. Having a will makes the court hearings easier and faster.
If you have kids who are still minors, you can specify who should take care of them
Disinherit people who would otherwise have a claim. If you have an ex-spouse or estranged children they may have a claim to your estate unless you specifically disinherit them
