Because according to the provisions of the Tax Administration Law and the Measures for the Administration of Invoices: units and individuals collect money from outside for selling goods, providing labor services and engaging in other business activities, the payee should issue invoices to the payer. "Should" means "must". If the terms in the contract violate the laws and regulations, then these terms are invalid.
In addition, it is illegal to invoice without adding tax points, but invoicing will increase tax points. It may be that the other party uses the income that is difficult for the tax authorities to grasp without invoicing, and does not declare and pay taxes to the tax authorities, which is illegal.