If we are not earning enough we can go elsewhere

Employers compete for their staff...

We can make money by helping someone else, but is there money to be made from someone who has no wants? What do the poor have to give?

If we can provide for a person, so that they may spend their time on a more pressing need then we have served the market.

For example, if we provide the tools which a craftsperson requires to do their job then we will make a profit, mutually. We enable others to better accomplish their goals. If we sell the tools to the craftsperson who serves us then both people gain from the arrangement. We help each other to remove problems.

But our assistance may be contingent on reciprocation. This differs from charity in that there is a cost to us performing our leg of the trade. We would rather retain our tools and the craftsperson would rather do nothing than perform the task we request of them. We do not mind giving to charity, but we would prefer not to be compelled to show up for work.

The volunteer is doing work which they value, they might not value the (other, normal) work they perform for pay, at least not to the same extent as it is valued by their employer. If the work is valued by the one performing it, then we are working for ourselves. We can think of our pay as a donation in gratitude, paid to encourage us to continue...

If your work is valued then people will compete to acquire it. They are paying you to do it again, in a sense, from the point of view of the worker but the work is already done. In most types of work, where we are not selling a product, but our expertise or effort, it is too late to withdraw our work once we have completed it. In this case we require our employer to pay a premium for good work to retain us, not to make us do the work in the first place.

If we have nothing to give, then we rely on charity, which is to say that we rely on others valuing (the thought of) our continued existence.