Ski warfare had a number of advantages. But what it really boils down to is that ski troops were incredibly agile, could traverse difficult terrain, used guerrilla warfare, set up ambushes, and get to better vantage points. They were also silent. Skiing allowed for Finnish troops to travel in and across places that would be difficult or time consuming to travel without skis. They could out maneuver vehicles such as tank, since they were slower, and vehicles better suited for roads. Their training gave them insight on how to use the terrain as cover to set up ambushes. It also gave them the ability to best judge where to set up for vantage points.

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Here is just a little bit of information on Finnish ski troops and some of the things they were able to pull off.

During the Battle of Suomussalmi, Finnish ski troops were able to disable advancing mechanized troops. Thus allowing for the three Finnish regiments consisting of 11,000 soldiers to overcome and destroy the two Soviet mech divisions made up of 45,000 soldiers.

Link to some more info.

In the beginning of the Winter War, in Karelian Isthmus's forests, 1,000 Soviet tanks were annihilating Finnish forces. Ski troops were able to out maneuver the tanks and get so close that they were able to use molotovs and charges to disable the tanks. "Finnish ski troopers, quick and agile in the forests, wove through the trees, using their white uniforms to remain concealed in the snow. The skiers tossed Molotov cocktails and satchel charges through the exhaust opening into the tanks’ bellows, causing the vehicles to explode from the inside out. In one instance, a Finnish ski trooper sledded close enough to pry the treads off one T-28, demobilizing the tank and allowing other Finnish skiers to plunk explosives inside."

Additionally, because of their training, Finnish ski troops had a incredible understanding of how to properly utilize the landscape to their advantage. "On Dec. 6, 1939, Stalin’s army mounted a large-scale infantry invasion near the Taipale River. The Soviets, having a huge numbers advantage, plowed through the snow towards the enemy. But the Finnish ski troopers, again utilizing their knowledge of the white and wooded landscape, expertly positioned automatic weapons that mowed down wave upon wave of advancing Soviet soldiers."

In an area of the Finnish front known as Summa and Lahde. "The Soviets used flamethrower tanks to scorch the Finnish trenches while the Finnish army fought back fiercely. It’s been said that two machine gunners fired 40,000 rounds between them. In the evenings, the Finnish ski troops counterattacked. By Dec. 21, 1939, Stalin’s birthday, seven Soviet infantry divisions had been wiped out along with 250 T-28 tanks."