The default value of a block is generally a common value that most people won't have to change. For example, the change () by () block has a default value of 1. This way it functions as a score keeper, timer, or counter. They can also serve to show how the block functions. For example, the Join block has the default values "hello " and "world". When a user sets a variable to the block to see the output, he or she sees "hello world", which explains the function of the block (concatenation) instantly.
A boolean insert's default value is "false", even though they don't show their value. This can be tested by simply running this script (notice the empty Boolean insert, where we make the block use the default value):
when gf clicked if <> then say [True] else say [False] //The sprite would say this. Try it! end