From the article:
During the Winter War (30 November 1939 – 13 March 1940), the 81mm mortar played a vital role due to a lack of field artillery that plagued the Finnish Army throughout the conflict.
The 81mm mortar was able to breakup Soviet infantry attacks due to its surprising effectiveness even in deep snow; due to the sensitive fuses that were fitted to the projectiles. However, field artillery they were not and proved less effective against bunkers and reinforced dugouts.
During the Continuation War (25 June 1941 – 19 September 1944) the number of 81mm mortar in Finnish service peaked but the attrition of war meant a steady decline in the numbers available. Any captured Soviet 82mm mortars were quickly turned against their former owners but the demand for the 81mm mortar was far in excess of what was available.