Coinbase provides web and mobile Bitcoin services. The mission of the founders is to provide Bitcoin value added products for consumers and merchants. Their users deal with the company rather than directly with the network. Coinbase is recognized as the PayPal of the Bitcoin Network.
Coinbase is more like Joe's Cut-Rate Check Cashing next to the liquor store. Merchants don't keep money in Coinbase for more than a few hours. Bitcoins go in, and US dollars are deposited to the merchant's account that night.
You
can keep money in Coinbase, but shouldn't.