Because, presumably, the program wasn't written as one big main() function, and at least one of the exit()s wasn't in main() — hence the use of longjmp() to jum

Because, presumably, the program wasn't written as one big main() function, and at least one of the exit()s wasn't in main() — hence the use of longjmp() to jump out of all the layers of function calls back to the caller of the `library'.