I use separate powder, salt and rinse aid and if I get one of them wrong it's very obvious. I feel I know exactly what each one does.
I can only speak for my dishwasher but it has no ability to sense how hard the water is or how much of each should be added. It does know when salt or rinse aid runs out and it will turn on the red lights to indicate this. It has a built in water softener which it uses to soften most of the water that it uses. Salt is only used periodically (perhaps once a week?) for regenerating the water softener.
The manual says that the hardness of my water should be looked up and then programmed into the machine. I looked up my water hardness on my waters supplier's website. It's a one off job.
If the salt runs out then the water softener beings to work less well. And then everything gets coated in a fine film of white limescale after every wash. It's a pain to get off. I live in a hard water area.
Rinse aid is less important. It encourages the water to flow off after it has finished and prevents drying marks. I feel it also coats everything with a film making things sparkly. I think there may be a bit of rinse aid in the sainsburys powder so I dial down the rinse aid setting on the machine.
So, the salt is used to regenerate the water softener. It never goes near the plates. Rinse aid is added to the machine to help drying. If you live in a soft water area or have a separate water softener then it is very possible you won't need salt. You might be able to turn off, or dial down, the water softener.
I don't fully understand how the pods/tablets work. They certainly won't have salt in them. They must have an alternative technique for preventing limescale. I feel I get better results with the separates and have fine tuned exactly what my water needs. I also feel it is gentler on my glass and plates.