"If you’ve ever built a theme, you’ll know about “the loop”. It’s the way WordPress exposes data to themes, in the form of a PHP API, and lots of themers find it frustrating. Instead of WordPress saying, “here are the posts you wanted, do what you like”, it makes you work within the loop API, which drip-feeds posts to you one at a time.
WP-API completely inverts that. You ask WordPress for the data you want — say, the first ten posts in May — then what you do with it, and how, is 100% up to you."