Show/Hide are very useful for a variety of reasons. It might even be possible that 90% of scratch projects use it. You can show and hide sprites to develop story, create humor, and to minimize things on the screen.
The say and think blocks are very useful for telling a story. In fact, it is one of few blocks that can develop a full story.
The visual effects like color effects and size can be very important when first starting out a project. If you want to adjust the size of a sprite, you have to use the size blocks. The color effects can be used for certain games or stories.