Hill Trolls, Olog Hai, Half Orcs and Goblin Men: These cards let you spend the die to muster only after you discover you need to muster. Since the cost to walk reinforcement armies to the sight of a stalled siege is so high, one normally over-allocates forces in the initial strike, sometimes by as much as double the necessary force.
For example, sometimes when assailing a pitiful [3/0/0] Dol Amroth, an extra three elites drop out of hyperspace on you, and life gets stressful if you only brought [6/2/0] Southrons.
Hill Trolls and friends free you from this.
Proably the ultimate use is going against a tough stronghold - say [3/2/2], with 8 regular orcs. With the Witch King and good combat cards, its about 20% likely you'll blow their doors off with just the 8 orcs. In that case, great. Its 40% likely you'll stall out and can get back into it with the Hill Troll cards. And its 40% likely you'll get your but handed to you so badly in the first rounds of combat, that the trolls wouldn't have helped even had you played them.
So 60% of the time you saved yourself the dice for mustering the trolls. The rest of the time you know those dice were really needed to win the siege, which is a warm fuzzy feeling.
Plus these cards are great when defending against Free People Military Victory.
I typically utilize the cards when I intend to finish a siege that I don't have the elites for. Worst case scenario, I win too easily and then have a more powerful army than I though continuing on to the next victim region.