JDBC driver, for example, use static blocks to register themselves to the DriverManager. That's why loading a JDBC driver by Class.forName initializes it.
Another use is to initialize complex static variables, such as Collections etc.
They shouldn't be used too often, though - most often there are better (more flexible) alternatives to static blocks.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus