They are released in preparation for 3.0, and the most relevant change is that they include support for the SVG 1.1 DTD in DefaultEntityResolver. Version 2.0.6 is intended to be the last version of the 2.0 branch (unless a security problem is discovered).
I suggest that users skip these releases and wait for 3.0.0 which is coming real soon. Version 3.0 splits XML-DTD as an independent module.