What is the purpose of adding salts to kettle?

Water adjustments to the mash are part of my brew day to achieve correct pH and residual alkalinity but not sure about additional get to kettle

Depending on the water, a certain salt/mineral addition to the mash would put the pH out of range. In these instances people will add the salt to the kettle instead. This way the overall water profile is at the desired ppms and the mash is at the desired pH

Kettle additions are mainly for providing the desired flavor profile to the water. They may also be for boosting the calcium content to improve yeast and fermentation performance when that calcium was left out of the mash in order to promote a proper mash pH.

Brewing salts generally do three things, Ph/RA, add appropriate minerals for yeast health, and the SO4/Cl ratio enhances hoppiness/maltiness depending on what end of the spectrum you're at. I will start by adding appropriate amounts of gypsum, epsom salt, and CaCl to get the right SO4/Cl ratio and the minimum amount of Ca and Mg. If that addition is going to lower Ph too much for the mash, I'll scale it back and add the remainder to the kettle. It's pretty rare for me though. I'm at less is more as far as water chemistry goes, so I don't add salts just for the sake of getting to some city's profile, but I imagine the folks that do that likely have to add to the kettle.

I too have trouble understanding water treatment. My water has 82 ppm chloride and is hard, so after adding AMS and AWB I should get 228 chloride (target 150/300) and 417 sulphate (target 250/400) . Can I just take out the hardness with sulphuric acid, mash with added calcium sulphate and add more of it to the boil? This would leave the chloride at 82 so I could also add a little calcium chloride to the boil. My beers are always pale hoppy bitters.