I came over all unnecessary yesterday!
Mr Tesco sent me some vouchers. That seems to happen quite often these days. He send me a voucher to spend if my shopping basket exceeds a certain amount. In this case I could spend £50 and save £7 but I have to get the order on or before 2nd October. I went into on-line overdrive and managed to sort a basket of £50.07 which would cost me £43.07 (plus £1 delivery) so I would "save" £7.
But I wouldn't really save anything. I would spend £43.07. True some of that would be things I need like eggs, bread, butter, One way or another I really will need those things very soon. Soon after that I will need pasta, olive oil. In the not-too-distant future I will need Braeburns, Stilton, celery and tomatoes. (OK, Stilton might not be a necessity for you, but it keeps me happy.)
You get the picture. When I sit down coolly and rationally, I can see that rather than a £50 delivery from Tesco a couple of much smaller spends from Lidl would save me far more than £7. Yes, in a few weeks I will need a delivery. But not yet. The moths can stay in my bank account for a few weeks longer. Even Mr Tesco sent me £5 vouchers to use next month if I need to.