Experience

What does it feel like to fire a 9mm?
How difficult is it to turn someone onto his stomach and handcuff him?
How does it feel to be punched in the nose?
Pepper sprayed?
To fall madly in love?
To have your heart broken?
To be stalked?
To have someone shove a gun in your face?
To be mugged?
To lose a loved one?
To save a life?
To do CPR?

Do life experiences such as these make you a better writer?

Do they allow you to more accurately and realistically write about such things?

If you've never been a spy working the streets of Tehran, can you accurately write about it?

Just some thoughts...