According to British WW2 Historian Richard Overy, the point of bombing Sofia was to take Bulgaria out of the war by demoralizing its civilian populations and adding to the anti-war rhetoric that was springing up by 1943 in Bulgaria. While Bulgarian forces were not involved in any major invasion, its [territory] was used by the axis for their campaigns in Greece and Yugoslavia, and Bulgaria anti-aircraft machinery did shoot down Allied planes that were targeting crucial oil fields in Romania. Bulgaria was still a (albeit minor) part of the axis war effort, and bombing was viewed as a relatively low risk way to force the nation out of the war. As Churchill stated regarding Sofia: "the effect of bombing a country where there were antagonistic elements was not to unite those elements, but rather to increase the anger of the anti-war party.” Furthermore, Sofia was both densely populated and heavily utilized by the Germans for transportation, making it an idea target to both harm Axis material and demoralize the civilian population of Bulgaria.

Edit: incorrectly stated Bulgarian forces were used in the Greek campaign