The Chart of accounts is recommended to be consistent for GL and Activities. The Activity field, when populated, already differentiates the JE entry line from one to the same account without an activity specified. This allows for easily determining how much was booked to a specific account and also split the bookings to a specific account into project related or not.
Account Categories in the AC module also allow you to have a separate chart of accounts, if desired, from your GL Chart of Accounts.
Since an account category is required when an activity is entered, it allows for a separation of the GL and AC charts if separation is desired.
Many users default in the Account Category so this value doesn’t have to be specified on each transaction that uses an activity throughout the system. The defaulting could occur on GL20 instead of GL00 if the accounting unit used in the transaction would alter the way you would account for the project/activity posting.