You need a will if you're married to declare who gets your stuff upon your death. Not all states work the same. 

Protect yourself and your spouse.  

If you have kids, you really need a will.  After your spouse, your kids should get everything.  However, what kiddo gets what is something you should want to document.  Don't leave anything to chance. Your state may not agree with your intent, 

unless it's in writing.

If you're single, no kids, no stuff, pause for the cause.  It's a good idea to get one when either situation changes.  

Seek a professional estate planner to draft your will for you.  Don't assume a shortcut way will hold up in court.  

One more thing: Make sure you update your will as situations, assets, family members change.