A few Premaster candidates feel their English is already quite good. Thus, they ask, Why study for the Verbal section?
A better question would be, What return can I expect on my study? Is it wise to invest so much time chasing better math scores, when "easy" points are available in English? Don't forget that your overall score is built from both sections... so it would be good to know where the easy points are.
The way to do that is to check your percentile ranking for each section. If you are below the 60th percentile in verbal, then you have reason to study those topics, instead of pouring in the effort to raise a quant score to the 80th percentile. (Those of you who want to understand how this works, then - without getting too technical - suffice to say that GMAT scores are scaled to fit a normal distribution, so it takes less effort to go from 50th to 60th percentile than from 60th to 70th.)
If you do find it valuable to study grammar and logic, then you may feel it can be done solo. And indeed it can.... only your return on investment could go into free-fall. In contrast, the Dojo has built a course in grammar and logic that could easily add 100 points to your overall GMAT score - one month, fully guided, no unpleasant surprises. Plus, the price is so low that your ROI goes through the roof.