I agree about Nectar points - I only use them as I find it entertaining to confuse their machine learning as I use someone else’s Nectar card.
However, if you combine something like Flux: The End of Paper Receipts & Loyalty Cards then the effort becomes zero (in theory).
You link your Nectar card, it gives you points every time you shop without you doing anything.
The offer from Sainsbury’s to those Monzo sees shop there but do not have a linked Nectar card could then be a ‘get x extra points if you link your Nectar card to Monzo’ - as Sainsburys want more people to actively use the Nectar scheme. If you don’t want to you dismiss and ‘don’t show me again’, but in theory you do it once and then get some value each time you shop with zero effort.
Or Sainsbury’s experiment with actually offering minimal pure cashback - again small because of aim to retain not to win, but still has value to the customer. I suspect Sainsburys would not want to try this though.
Getting into too much detail - my point is that Partners do have small reasons to attach their own offers to their own customers.
Key word, small.