When should I scrap my projector for a blackboard? These days, we've got all this fancy, new-fangled technology. We've got live coding, we've got presentations, we've got remote desktop adapted for classroom use. There seems to be a tool for every teaching problem. But when should we ignore the KnowledgeInjector2000TM and instead use the good old blackboard?

More specifically, which concepts are easier to explain with a blackboard and how would you use a blackboard to explain them?
Shifting to the blackboard/whiteboard from the projector serves two main purposes:


Slows down the pace of instruction. I can type much faster than I can write on the whiteboard. However, this is not always a good thing. When I'm discussing something complex or want to ensure my instruction does not move too quickly, I shift to writing things out. This forces me to be more thoughtful with timing and often times has prevented me from giving students more information than was necessary for a particular lesson.
Shifts focus from code to concept. As I say in a lot of my answers, differentiation is key. Staring at code to understand a concept is not always going to make it sink in. By writing on the board, I often times can draw close attention to a concept (i.e. recursion or variables) outside of its context within a program. When introducing something new and/or something complex, this helps me ensure that students understand the thing before they code that thing. For example, with recursion, I draw out stack frames, naming each one with its function name and arguments and drawing arrows with values passed in and values returned. The magic of a few lines of code that calculates a factorial becomes a little less opaque when there's a visual abstraction to which students can connect the code. Additionally, introducing variable assignment to beginners, especially something like x = x + 1;, works well on the whiteboard where I can number the steps, translate it into "English" underneath, use different colors, etc. 


Bonus: It's not always a game of either/or. Projecting code onto a whiteboard mixes the modalities: students can see "real code," but that same code can then be analyzed and annotated by hand.

For my own unique context in a classroom with whiteboard-top desks, I can have students write down what I write down when I use the whiteboard. It becomes a shared task since we are all doing the same thing and forces me outside of the "sage on the stage" paradigm for a minute. Instead of lecture and fast-typing, I can participate along with them in writing on a whiteboard. This has as much to do with engagement and classroom environment as it does with pedagogy.