Firstly, Coinbase is insured.
Digital Currency
All digital currency that Coinbase holds online is fully insured. This means that if Coinbase were to suffer a breach of its online storage, the insurance policy would pay out to cover any customer funds lost as a result.
The insurance policy covers any losses resulting from a breach of Coinbase’s physical security, cyber security, or by employee theft. The policy is provided by a syndicate of insurers through Lloyd’s of London.
Coinbase holds less than 2% of customer funds online. The rest is held in offline storage.
This insurance policy does not cover any losses resulting from the compromise of your individual Coinbase account. It is your responsibility to use a strong password and maintain control of all login credentials you use to access Coinbase and GDAX.
https://support.coinbase.com/customer/portal/articles/1662379-how-is-coinbase-insured-
There are also dangers to keeping your own bitcoins on your computer.
How safe is your computer?
What happens to your bitcoins if the hard drive crashes and burns?