Purpose of the product
Part of the Skype external access/ Mobile deployment includes opening access to the internal Exchange in order to allow Skype to view Calendar info. This is done by a services named EWS (Exchange Web Service)
EWS protector is designed to restrict access to the EWS services only to devices that are accessing from Skype for Business and from a managed device.
The risk handled is that someone can get access to company Exchange via EWS by only having user name and password.
Note that some systems require access to EWS which are not from S4B . You can add some exceptions to the filter in order to bypass the EWS control
More information is available here:
https://agatsoftware.com/skype-for-business-exchange-web-services-protection/