The update Clients are often operated by human users. Since an update normally is a long process, the user would like to see the current state. At a first glance the percentage can give a hint about completion of the update, especially if several devices are updated at the same time. But if there are unexpected delays or errors the user needs a detailed textual description about the current update action or issue.
This can be accomplished with the UpdateStatus Variable (see 18.104.22.168). A Client can subscribe to it for a user display. At least if a state machine is in an error state the UpdateStatus should provide a meaningful error message for the user.