As the Clone Wars raged on, Galactic Republic army commanders realized the need to build up and diversify their walker armor. One of he new designs was the AT-AP, a highly effective means to transport heavy artillery from place-to-place. Its main function was to serve as a mobile gun platform to shell enemy locations. Although armored, the AT-AP was not designed to function autonomously, and often required support from heavier units such as the AT-TE, Trident, or A6 Juggernaut. Many of these units were used during the final days of the Clone Wars on heavily-defended Outer Rim Separatist strongholds. Some were also used during the defense of Kashyyyk.
During the Galactic Civil War, the Empire slowly phased out the AT-AP in favor of more independent units such as the AT-ST. Abandoned factories formerly used to produce the walkers fell into the hands of Rebels and pirate organizations. AT-APs would continue in Imperial service on the planet of Kashyyyk, where they were used to quell the frequent Wookiee uprisings.
The AT-AP is a heavy anti-vehicle unit. Individually it fares quite well against most vehicle types. The primary cannon deals incredible damage, able to effortlessly destroy a T2-B in one volley. They also have very long range, which is fitting considering the "Sniper Tank" nickname. However, they only fire twice every six seconds and unlike the TIE Crawler, they don't have a secondary weapon. They are heavily armored, but that isn't enough to save them from a swarm of Freerunners. They are also fairly slow, barely able to outpace an AT-TE.
Overall, AT-APs are good for taking down large targets from a distance, however they fall victim to large numbers of smaller targets.