If your DITA2Go project includes one or more ditaval files, DITA2Go can use the content of these files to modify or annotate conversion output. In other words, if you have an existing project, perhaps using the DITA Open Toolkit, you are not forced to change to DITA2Go proprietary settings. As an alternative, DITA2Go provides configuration settings that replace and extend the functionality of ditaval files; see §9.2 Defining conditional actions.
If you do not have an existing project, you can go either way, depending on familiarity and specific project needs. If you are still using the Open Toolkit to generate PDF output, for example, you might need ditaval files for that, and you may find it easier to tweak a ditaval file for HTML than to redo its provisions the DITA2Go way.
§9.1.1 Specifying a single ditaval file