Why is it safer to keep your bitcoins offline than online on Coinbase?
Let's assume you keep your bitcoins on the exchange for 5 years. If the company behind the online wallet goes bankrupt, is hacked, or is malicious, you lose your bitcoins. If you kept them on a hardware wallet and kept the mnemonics in another, safe location, you can still recover them in case of fire, disaster, turmoil. 

Mt.Gox taught us that trusting in another's private keys for your holdings is not a good idea. Many of the people who lost hard in that episode are still waiting for a reimbursement.