you can add a computer account but SEP will not be automatically installed on it. When you install SEP on the client using the deployment wizard, it is automatically registered into the management console.
Maybe, in case you deploy a SEP client in user mode, adding a computer account has a bit of sense but in the most of cases it is not so crucial or useful.
The main purpose of "Add Computer Account" feature is when you are having clients installed computer mode and user mode.
You can configure clients to run in either user mode or computer mode. In user mode, the client computer onto which a user logs on uses the policy of the group to which the user belongs.
In computer mode, the client uses the policy of the group to which the computer belongs. The applied policy is independent of the person who logs on to the computer.