A client record here carries the things that actually change what happens: their state or region, which decides the tax on every invoice; their tax number, checksum-validated where we know the format; their currency; and their payment terms.
Everything in one place
Open a client and you see their invoices, their proposals, their contracts, what they owe you and how late it is, and what you have collected from them all time.
Concentration, said out loud
The reports will tell you what share of a period's revenue comes from each client, and say something when one of them is more than half of it. That is not a problem until it is, and it stops being one only by winning work somewhere else.
Archiving, not deleting
A client you no longer bill can be archived. They disappear from the pickers and stay on every document that already referenced them, because deleting a client who appears on last year's invoices would quietly rewrite last year.