The CRM, the accounts package, the online store, the SMS provider and the spreadsheet each hold a version of the customer. Integration makes one of them the source of truth and keeps the others in step, so nobody types anything twice and the numbers agree.
The hard parts are field ownership, stable identifiers, retries and reconciliation, not the connection itself. An integration built without them works on the demo and drifts in production.
Our integrations page lists the 52 systems we build against, from payment gateways and CRMs to accounting packages and messaging tools.