First and foremost if you don't have an estate plan someone else decides what happens to the fruits of your lifetime of labor and love.  That someone is usually the state, guided by its laws on intestate succession.
Second in many states there are estate taxes whether or not you are sufficiently well-heeled to worry about Federal estate taxes.  So an estate plan allows you to address issues of tax minimization for your spouse or significant other and your children.
Third if you have cherished possessions that you want a particular family member or individual to have when you're gone, you need to make certain they will get that cherished possession.  You shouldn't trust other people to read your mind or be altruistic when it comes to possessions having sentimental or great financial value, you're not around to say wait a minute I wanted so-and-so to get that.  
Fourth as you age your circumstances change and having an estate plan allows you tailor the disposition of your assets as your circumstances change.  As we get older we get fatter or we loose weight and we need our clothes altered, that is also true of estate plans.
Fifth making an estate plan gives you a chance to discuss what your wishes are with your loved ones and gauge their reactions and consider their feelings while they are not raw from your sudden departure.
Sixth it makes matters easier on those you leave behind.