You have the option of creating a flight with no impression goal by selecting No goal when creating a flight instead of entering a goal value.
This turns the flight into an unreserved inventory flight that is not taken into account in forecasting as a competing flight. Since No goal flights do not reserve inventory, they have no effect on the impression goals of other flights. However, a No Goal flight can affect the delivery of other flights with the same or lower priority. For details, see How Ad Selection Works.
Note that Pacing options are not available when No goal is selected.
Reasons for creating a No goal flight can include:
The flight is a filler flight.
You want an always-on flight that does not reserve inventory. This is a typical use-case for a sponsorship flight, in which you want the sponsored flight to always have top priority. If you do not select "No goal" with a sponsored flight, the flight will only function as a top-priority sponsored flight until its daily goal is reached, at which point it will stop delivering for the rest of the day.
While not a requirement, Triton Digital recommends you create No Goal flights within their own campaigns instead of mixing goal-based and "No goal" flights in the same campaign. Doing so makes reporting and OTI readings easier to understand.
Selecting No Goal can affect how flights with Programmatic priority type deliver. For more information see Creating a Flight.