Purchased from Germany in order to fight back against Russian armor, the anti-tank gun helped slow the tide of the Red advance. However, due to the heavy weight of the gun, it wasn't very maneuverable, and thus if an emplacement was to be overrun, the Fins just sort of had to leave the gun there.
And though not often talked about, the Russians did have an air force that needed to be defended against with flak guns, particularly around military bases.
From the website:
Finnish armies gain some welcome firepower this week with the arrival of these two German-supplied guns – the Finnish 75 PSTK/40 anti-tank gun and the Finnish 20 ITK/38 anti-aircraft gun.
After encountering T-34 and KV-1 tanks, the Finns realized that heavier anti-tank equipment would be required. A total of 210 German 75mm PaK 40 Guns were purchased, capable of piercing even the thickest Soviet armour of the time. They were extremely accurate and powerful weapons, but due to their heavy weight they were difficult to move quickly.
With relatively few tanks to choose from compared to their Soviet foe, the Finns relied heavily on the anti-tank capability of guns like this.
Despite their secret agreement with the Soviets, the Germans delivered a total of 50 20mm anti-aircraft guns for the Winter War and around 100 FlaK 38 guns were supplied from Germany for the Continuation War. These totals were a mix of static and mobile weapons with the majority of the static guns being used by naval and coastal defence forces.