Lots of good suggestions in this thread:
Remember the LotR scene of Helm's Deep, where repeatedly the actions of a few defenders have impact on a desperate battle: holding out long enough for the reinforcements to arrive.
Another important task, as mentioned before in the thread, is the attrition of battlefield supplies, a task highly suited to an elite team of brave, and daring adventurers, bold enough to operate behind enemy lines, and a danger to the enemies rear so great, as to require a large number of forces to protect the supply depots, and chase / harry the partisans. Armies do not just arrive on the doorstep, but take years to gather supplies, gather strength, train, organise and plan, and months to march across terrain, or weeks in ships (but ships need to be planned, suitable forest found and harvested, built by professional shipbuilders, crews trained). None of which can be done in particular secret.
Consider the Persian army in 480 BC. Said to be the largest army assembled at that time, some chroniclers put it at 1 million (which, if true, probably included baggage train, animal handlers, support engineers, craftsmen, the friendly women-for-hire, cooks, blacksmiths, etc). No wonder the Greeks knew the Persians were coming. They knew the when, they knew the where. Didn't save the city of Athens from plunder. But the King Leonidas of Sparta bloodied the nose of the Persian tyrant Xerxes at Thermopylae, to inspire stories thousands of years later.
Demonstrating the contribution the PCs make to the fight can be in terms of holding out until reinforcements arrive, blocking an area until the old, young, sick, and infirm have escaped or are rescued, recapturing an important hold, supply depot, or fortification. It could also be noticing important events and reporting to commanders in a timely manner, to another allow a unit to withdraw against impossible odds (saving lifes), or to reinforce a cracking infantry unit with the Elite Dwarven Squad in the nick of time. It could also be by carrying important messages across a confused battlefield to isolated commanders. Or rallying troops by standing by their side when the odds seemed stacked against them, and their sergeant just croaked.
By describing the effect of the character's actions on those around them; the way they look to their heroes for guidance, for hope. By the flash of fear in their opponent's eyes.