The ease of use, how it integrates with other systems such as Salesforce CRM or Office 365/Sharepoint etc., that it's cloud platform can be used from anywhere at anytime without having to worry about being offline.
As mentioned above - integration capabilities are limited when compared to competitors like Zoho Business Suite for example but this does not mean you should avoid using Merit in your business! It works well enough so far I'm happy with my decision. We're solving problems we have within our team by moving all our communication channels into one place (including email).