Explanation
The gap between organic-only usage and perpetual paid usage with exclusivity is often larger than the production fee itself, because running a video as a paid advert for a year is a fundamentally different product from posting it once. A quote with no stated usage term is not a quote, it is an ambiguity that will be resolved badly later.
This is also the axis on which generated content differs most sharply. Content you generated has no external licensor and no expiry, which removes a category of ongoing administration that scales badly with volume.
The common mistake
Assuming payment transfers ownership. In most jurisdictions it does not by default, and running content past its licensed window is a straightforward breach rather than a technicality.