How comments work on Pinterest
Pinterest is closer to a visual search engine than a feed. People find Pins by searching, save them to boards and come back when they are ready to cook, build or buy. Comments are a small part of that, which is why many people go looking for how to see them at all.
When there are comments, they sit under the Pin. The person who created the Pin can reply, and other planners read them for tips before they try the idea themselves.
That makes the best Pinterest comment a practical one: "I made this with oat flour and it held together" helps the next reader more than "beautiful".
Commenting as a brand or creator on Pinterest
If you publish Pins yourself, answering the comments on your own Pins is worth more than commenting elsewhere: those comments come from people who saved your idea and are about to act on it.
On other creators' Pins, add something useful and leave links out. Pinterest filters comments with links, and the creator is more likely to answer a comment that adds to their Pin.