What if I'm asked to create a vertical bottom wall that will transition into a slanted upper wall? Initially the wall configuration looks like the walls on the left in the following image. I added another vertical wall at the top of the slanted portion just for fun.
Each wall doesn't know anything about the other so there is no attempt by Revit to miter them together. We might have expected them to join? If we consider what happens in plan views for a similar layout (see image) I don't think it's unreasonable to think that might have happened. If an automatic join occurs in plan views maybe using Join will work in a section? No. Wall Join tool? No. Attach? YES!
To resolve the clumsy look of the left walls and end up with those on the right side of the image we can take advantage of the Attach tool. We just need to add reference planes to define where the miter joint should occur. Then Attach - Top will fix the bottom wall and Attach - Base will fix the upper wall. Repeat as required until all walls relate to each other better.