Electromagnetism: Trouble understanding the D field

I've just finished my first term in an undergrad electromagnetism course using Wangsness as a textbook and we ended on the displacement field.

I feel like Wangsness doesn't give a very good explanation of what the whole point of it actually is, and I still don't quite understand how to use it to find the e field inside both a dielectric and a cavity within a dielectric.

If I could get a quick and easy explanation or a point in the right direction I'd greatly appreciate it, thanks!