I think a tax lawyer would have to answer this, and that it probably depends on the country.
In my country, I (even if "I" means a company) can receive a business gift with a maximum value of 42eur, twice per tax year from one company - everything above that is taxed.
Let's say i did some work for a local store and charged them 2000eur for my work. To do my work, I had 1000eur of expenses. I pay tax on 1000eur difference. With eg. 20% income tax, I'd have 800eur of profit. (yes, a bit simplified, I know)
Then they decide to give me a 200eur coupon for their store and give it to me (to my company), and this is counted towards my income (2200eur now). If i actually need stuff from their store, I use it, so now I earned 2200eur gross, spent a 200eur coupon in their store and 800eur anywhere, I have 1200eur of profits, and even after tax I earn more than I did before - with 20% income tax, I'd have 960eur of profit in cash form.
On the other hand, if I don't need their services, and can't sell the coupon to someone else, the situation is different. I still got 2200eur of income (2k+200eur coupon), spent 1000eur in other stores, get taxed on the 1200eur, technically still have 960eur of profit, but 200 of those are in a form of a worthless coupon, and only 760 in cash - so their usless coupon is actually costing me 40eur.