We made a mistake and bought a bunch of Walmart butter several years ago without trying it first. Yuck!!! Never again. We do like the Aldi butter. Have never tried Kerry Gold.
There is more to salt (sodium) than just taste. If you are on a salt restricted diet due to Heart Failure or Kidney issues. On a 1500mg daily low salt restriction, even small amounts of salt in foods are still worth considering and substituting with alternatives.The amount of salt in "salted" grocery store butter is minuscule. Listening to a lot of youtube cooks and reading old cookbooks, I also thought that you -had- to cook with unsalted butter and judge the amount of salt needed yourself. But outside of some imported or boutique butters, the amount of salt in modern butter is not worth fretting over.
Decades ago, salt was added to butter in massive amounts over what we use now. This was to help preserve it, but also added to the taste.