Inquiries vs Inbox

I understand the CRM tab > "Inbox" is for API-integrated channels (for me it's Airbnb only). 
The "Inquiries" tab is for compiling all messages from various locations.
What is the benefit in having them separated?  I need a "centralized inbox" to make sure no one slips through the cracks.  This seems odd to split them into two areas.  Can they be combined?

This is part of a long-term plan to integrate with multiple different communications channels.  But it's more complicated than you might think, because different channels support different functions.  For example, Airbnb internal messages don't allow HTML or images as emails do, and some other channels limit the length of messages.  Also, some channels have a higher expectation of immediacy.  So there will be ongoing work, additions, features, and changes in this area for the foreseeable future.