Helps the PH in soil, good calcium for plants, softens clay helps it retain water, contains trace nutrients, slow release, good for worms, helps soil break down micronutrients for plants to use.
I just get the stuff they use for laying hens it's like $15 for a fifty-pound bag, powdered would be a faster release. When I plant trees, I mix a solo cup full into the dirt going back in hole, on existing trees it can just be spread around drip line. I usually only ever do it once.
I've seen the biggest benefit using it with conifer trees and in the garden...also mix it in the dirt in my starter box for just grafted fruit trees that I'm healing in for a year.
Another really good soil amendment is gypsum; I use it the same...sometimes in conjunction with oyster shell, very similar and hard to overuse. I spread gypsum over my whole yard it is especially great in clay soils. Takes 2-3 years to see results but will stop ground cracking and make lawn look like a golf course.