They explain in their blog post:
Why does this matter? Scale. The value of any communications network increases directly in proportion to number of users — and when it reaches a point where it includes most of the businesses you want to interact with, something amazing happens
Tradeshift has expanded rapidly. In May of 2011, Tradeshift announced $7 million in funding from Notion Capital, and now includes the French Government, Vestas, and Kuehne+Nagel as customers.
The blog post also points out that as Tradeshift grows, they become a more appealing environment for developers and partners to create and sell apps on their platform.
This video was included in the announcement, which explains what this news will mean for QuickBooks and Tradeshift customers: