Ask a team of thirty people where the customer list lives and you will get three answers. The CRM has one, the project tool has another with different spellings, and finance keeps the real one in a spreadsheet because that is the only version with the billing address.
None of those tools is bad. The problem is that each one owns its own copy of the same four things: who your customers are, who your team is, what people are allowed to see, and what you are being charged for.
What sharing a foundation changes
When a quote becomes an invoice, nothing is re-typed. When a deal is won, the project starts against the same account. When someone leaves, one permission change removes them from everything.
That is the whole idea: the apps are different views of one dataset, not different products that occasionally sync.