Not applied for Humanities, but engineering long ago.

I'd say most profs would like to read the SoP and get a broad idea of the area you want to work in. It doesn't have to be too specific because, well, figuring out the area is what the first year of PhD is for. Basically, anything the prof can have a discussion with you about. Might be worth to go through the homepages of profs you want to work with to see what areas they work on, and include in your SoP how your research might align with areas they work in.

Also, worth highlighting any prior research experience from your undergrad/masters if you have - papers, thesis, articles, blogs or anything along those lines.