Here are three benefits that I am seeing from this system. One has been mentioned before here (at least in the comments), one I anticipated, and one I only realized this week.
First, I suspect that there may be some sort of a metacognitive boost with this grading system. Students are forced to reflect on what they have done, and this may be helpful.
Second, grading is much easier when students use different approaches. In a very real way, I am just grading whether their “tags” are legitimate (the are correct, relevant to the problem, and point to a specific part of the solution where it is relevant). This means that students can have wildly different solutions with completely different tags, and they will both get appropriate credit. This hasn’t happened a lot yet, although I imagine it could.
Finally, my new realization is that this grading system may do away with a lot of fighting over grades. For example, a colleague recently complained that when students are asked to “graph functions” in Calculus I, many students were doing so simply by plotting points. My colleague did not want to give them credit, since he intended for them to find intervals of increasing/decreasing/concavity/etc. The students were not happy that they did not receive credit.
This is not an issue in Accumulation Grading with Tagging. Students are welcome to simply plot points to graph a question, but they run into an issue when they start to tag their work with the relevant learning goals (there are none). But nothing is marked wrong (because it isn’t wrong), so there is no real disagree to be had between student and teacher.