To earn. That's why.
Marketing is moving the needle. It's gaining trust and earning attention. You want to either start the conversation or keep it moving forward. The idea here is to peak someone's interest enough to make a change, no matter how small that change might be. You are telling a story. If in some small way others connect with your story, then you are doing your job properly.
Having a hard time selling something? First, make sure it's something people actually want to buy. Second, focus more on your story and how you want to earn people's trust, attention, and permission than on what sales tactics you are going to use to make them buy.
That's how you build and sell something of value.