What is a typical use case for Account-Specific hours?
Organizations that have different entitlements or SLAs can use Account-Specific hours to move through the flag levels on a quicker (or potentially slower) schedule. For example, if you have Bronze, Silver and Gold support levels, you can configure the default hours with Bronze hours and mass update your Silver and Gold accounts with the respective hours.

Another use case is to reserve the 4th level with a unique icon to designate critical accounts.

Can I use Account-Specific hours for some accounts and not others?
Yes, you can configure Account-Specific hours on a handful of accounts, and cases for all other accounts will use the default hours.

Why set hours fields directly on Accounts and not have a level selector?
The hours fields on Account allow the most flexibility in how Case Flags works for each account. If you have negotiated non-standard entitlements or  SLAs with some customers, Account-Specific hours will allow you to configure them.

Can I standardize my Account-Specific hours and set pre-defined hours to named support levels?
Yes, if you have Enterprise or Unlimited Editions, you can create a custom pick list field with values for each of your support levels and use Workflow and Field Updates to set the CF hours X field accordingly. This approach also gives you the flexibility of having an Other option that doesn't use pre-defined hours.