While each Organization must find its own trigger, I’ve laid out some common underlying set of reasons which resonate across Organizations 

1) Productivity

• Repeated and redundant Tasks of creating, updating and maintaining data, especially when it is worked on by multiple groups within and outside the organization is done away with.  
• Setting notifications and alerts on relevant Portal activity, ensures users are informed of updates pertaining to their concerned areas. 
• Effective use of Dashboards can showcase synopsis of updated information 
• Allowing external users to create, update and view requisite information – data and documents, using inbuilt application validations and security, saves the Organization time and effort consumed to update it on the System from the shared location

Click on the link to view complete blog: Top 10 Reasons to Invest in a Dynamics 365 Portal