An error like the previous one, when part of a larger script, has the potential to completely alter a program's execution, leading to undesired and potentially costly results. In general, when something unexpected occurs during the execution of a program, it may leave the software in an erroneous state, making it impossible to return a correct value. In such cases, rather than pushing on and potentially propagating the problem throughout the whole execution stack, it's preferable to explicitly notify the calling code about the situation by throwing an Exception.
Many functions, both in Julia's core and within third-party packages, make good use of the error-throwing mechanism. It's good practice to check the docs for the functions you use and to see what kinds of errors they throw. An error is called an exception in programming...
