I only understand Salesforce in that they purchase any successful company they can, and take what seems like 30 minutes of effort for each acquisition to integrate it with the rest of their businesses in a sensible, synergistic way.
The original Salesforce, people now call SFDC (Salesforce Dot Com) which as you know is a CRM but more broadly speaking, a database backend & a GUI, with a construction kit to let ~customers~ Salesforce Consultants model and build simple tooling for business processes of any kind.
They have snapped up various companies like ExactTarget (bulk email), Demandware (aka SFCC, probably what's being discussed here), Heroku, and Slack.
I assume the main synergy is just of the business kind. You can include Slack in a big overall Salesforce "deal" and have one big (steak dinners and alcohol-style) contract negotiation.
And honestly even though I teased above about the lack of integration or consistency (it took like 10 years for one to be able to even log into ET with Salesforce creds), I'm glad that they don't really do anything to their acquisitions to integrate them, since I have no desire to have any integration between say, Slack, and our bulk emailing platform, and anything like that would surely make Slack worse.