On foot, your feet are everything. A blister that would be trivial at home can stop you walking, and in a situation where you need to cover distance that's a real problem. Hydrocolloid blister plasters do more than cover — they cushion the sore spot, seal out dirt, and create the moist environment that heals a blister quickly, letting you keep moving in far less pain.
They weigh nothing and cost little, yet they solve one of the most common and mobility-limiting minor injuries there is. A few in a first-aid kit or pocket is preparedness for the problem you're actually most likely to face on foot.