Artillery comps have one or two champions that will play teamfights by poking you and your team. Their defining characteristic is their long range and thus it makes it hard for annie to reach them and deal sustained damage.
Examples of these champions are jayce, ezreal, ziggs, varus (lethaluty build), and xerath
Rather, Annie wants to take advantage of the few moments she can access them so it is good to focus on her burst and mobility. This is why you would take rocketbelt as it gives magic pen and mobility from the rockets.

Thank you!