Caveat: the further you get into non-tech, as a customer base, the more prevalent this is. On the tech side, there's at least a thin veneer of objectivity (often masking a quiet steak dinner, in which the actual solution was picked).
The last negotiation I was there for, for instance, in late 2021, the SF guys were pitching (hard) a product called "Salesforce Einstein" which was promised to be everything to everyone. I see they are now calling it "AI" which wasn't the main way they described it then before AI was such a hot buzzword. I think it was just branding for some kind of marketing workflow tooling, but to hear them tell it, it would do anything we could imagine.
The current place I work will call up Microsoft, databricks, or someone else and have them pay for a hackathon with ~100 participants including catered meals and dinners out for a week.
I sat next to one vendor, I saw the total bill for the dinner (30k). He didn’t flinch and slapped down his Amex
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.