Right. If we provide teachers to an area, we might be eliminating the teachers already present. More likely we're supplementing them. This is filling a need that a local economy may not be able to sustain on its own.

If we provide volunteer laborer a to a farmer who can afford hired hands, we're reducing the income to those in the local unskilled labor market.

Those are two extremes and most labor-giving and goods-giving fall in between. But we need to determine where it falls to determine whether is a positive or a negative.