EventValues (i.e., the names that start with exclamation points and turn red) are different from EventVariables (which can be defined in an individual parameter field).
An EventVariable, like most variables, is a location in memory that you can read from or write to. EventVariables are local to the parameter field in which they are defined. An EventVariable can be assigned a single number at any one time.
An EventValue is more like the end of a pipe that is connected to the outside world. Data is flowing into Kyma through that pipe. You can route that pipe to any destination in your Sound; anywhere the !EventValue appears in any parameter field gets hooked up to that external source and the data will pour into that point. So the !EventValues are global in the sense that every EventValue of the same name gets the same stream of values. And it is read-only (the data are flowing out of the pipe into the parameter field, never flowing back into the pipe). In a sense, an !EventValue is more like a signal (a stream) than it is a single number or data structure.
The SoundToGlobalController is a way to generate an internal stream of data and hook it up to an !EventValue pipe. To a Kyma Sound, it looks as if the stream of numbers is coming in from an external controller or software, even though it is being generated in Kyma by a parallel branch of the signal flow graph.
In many ways, EventValues and EventExpressions are more analogous to Sounds and signal flows than they are to the variables of programming languages that do not explicitly reference time.
How's that for a long answer to a simple question? ;)