Difference between Adobe Pro w/ Sign and Adobe Sign

Hello,

I'm reaching out to gather some information on behalf of my employer. Currently, one department in the organization utilizes Adobe Sign, where they purchased a certain amount of signatures. I have used this platform before, and I believe it's all cloud-based, no desktop application for use. 

Currently, my department has Adobe Pro (primarily via desktop application) with the ability to send signature agreements just the same as I'm familiar with in Adobe Sign. 

My questions are: 

1) What are the differences between the two platforms? 

2) Are there any limitations in using Sign in Adobe Pro versus paying for a separate Adobe Sign subscription? 

3) Why pay for Adobe Sign if the org is already paying for Adobe Pro licenses for staff?