What causes Microsoft Outlook rule to be client-side only?

I can clearly see and confirm that adding and mark it as read flag to any rule causes my Outlook to start treating given rule as client-only:

While removing this flag causes that particular rule will be effective also in Outlook Web Access (at least that's I assume from these warnings):

So this is the ultimate answer to my question: Forcing any rule to mark any message as read causes that rule to work only, if offline Outlook program is opened.

However I completely don't know, why is this happening like that? Why simple operation of marking message as read can't be processed in Outlook Web Access. Must be a weird part of the a matter deeply in Outlook.

Server-side vs. Client-only Rules:

Client-only rules have at least one condition or action that uses an Outlook feature:

• Common conditions making it a client-only rule:
  • Assigned to category
  • Flagged for action
  • Marked as importance
  • Marked as sensitivity
  • With specific words in the body
  • With specific words in the message header
  • With specific words in the recipient’s address
  • With specific words in the sender’s address
  • With specific words in the subject
    
    
• Common actions making it a client-only rule:
  • Assign it to the category
  • Clear the Message Flag
  • Display a Desktop Alert
  • Display a specific message in the New Items Alert window
  • Flag message for follow up at this time
  • Mark it as read
  • Moving messages to a folder that exists only on the PC
  • Permanently delete it
  • Play a sound
  • Print it
    
    
• Outlook also imposes a storage limit of 32KB for rules in OWA