Preparing a QGIS-course for my colleagues I stumbled over the following question: What is the difference between the GRASS commands in the processing toolbar and the GRASS Tools, that you access via the GRASS toolbar?
To use the GRASS tools directly, you need to first import the data you want to work with into a GRASS database.
If you use GRASS algorithms through the Processing toolbox, you can skip this importing step because Processing takes care of all the data conversion. It's therefore much more convenient. You don't have to explain to your students how GRASS works, e.g. the region concept etc
Processing already contains a lot of GRASS tools but it is possible that you want to use one which is not included yet. If that's the case you either have to go back to GRASS or ask the developer if the tool can be included.