The default rules surprise people. Work created by an employee within the scope of their job generally belongs to the employer, but work created by an independent contractor usually belongs to the contractor unless a signed agreement says otherwise. "We paid the invoice" does not transfer copyright. Founders themselves also need to assign what they built before incorporation, or the company owns nothing it was formed around.
This is the clause that quietly kills deals. Investors and acquirers run IP diligence early, and a single unsigned contractor who wrote part of your product is a genuine problem — one that must be fixed retroactively, from a position where that person now knows exactly how much their signature is worth. The fix is procedural and cheap: nobody who creates anything for the business starts work before signing an assignment, and founders execute one at incorporation covering prior work.
A concrete example: an early designer builds your brand and marketing site for $4,000, invoiced and paid, with no contract. Three years later an acquirer's lawyers ask for proof the company owns its logo. It does not — the designer does, and they can now name a price or refuse. Had they signed a one-page assignment on day one, this would have been a line item in a data room instead of a renegotiation of the purchase price.