The SharePoint connector has an extremely useful action "Send an HTTP request to SharePoint", which can be used to initiate actions through the REST API, as well as other things.
This function is unique and special, because it leverages the existing credentials set up in the workflow for the user - no additional login token is required to issue the request (which would be required to use the standard "HTTP" function).
The Salesforce connector also needs to have this functionality added, in order to make the same functionality available to Microsoft Flow for Salesforce.
In particular, Microsoft Flow has no way to download a file stored in Salesforce (e.g. to pull it down and store it into SharePoint). Making the "Send an HTTP request to Salesforce" available will bridge this gap, and make files stored in Salesforce fetchable from within Microsoft Flow - as well as making other Salesforce API requests callable from Microsoft Flow, where they are not already available as part of the standard connectors.