Vegetables sprayed with water can absorb it across their surfaces, which will be absorbed by their cells. These cells will, in turn, become more turgid, making the vegetables crisper.
Work Step by Step
The thing here is to see that the vegetables are sitting in dry air in a grocery store, without roots to absorb water from soil. The dry air will pull water out of the vegetables, so the spray of water will reverse this and help keep the vegetables' cells turgid and the overall vegetables crisp.