Inactive users and deleted users still appear in analytics so you can see the source of where activities are attributed.

Keeping inactive user's data is particularly helpful if a previously active user sent an email to a Person and the Person opens, clicks, or replies to the email months after they’ve been inactive. Your team can keep track of the original message and continue the call. 

Inactive users typically appear in reports due to automation rules and/or records not being re-assigned to active users. The most common metrics that are populate after a user is made inactive include:

• Opportunities 
• People added 
• Emails skipped 
• Hot leads
• Calls dialed
• Linkedin connect completed
• Meetings cancelled
• Successes