That nice lunch might cost 20$ if you paid cash, but use your CC and it's really 20$ + the interest you are going to pay. If it's going to take you 4 years to pay off your 20% APR CC debt then that 20$ lunch is really 41$ 50c.
PS: That is not to say paying cash for lunch changes the numbers when you are in debt; CC increases the cost of "everything" you buy without exception.