Because on some systems (Linux/Unix) a new line is defined as \n while on others (Windows) it is \r\n. Depending on the software reading the text, it may chose to adhere to this or be more "forgiving" recognizing either or even \r individually.
Relevant Wikipedia text (https://en.wikipedia.org/wiki/Newline):
Systems based on ASCII or a compatible character set use either LF
(Line feed, '\n', 0x0A, 10 in decimal) or CR (Carriage return, '\r',
0x0D, 13 in decimal) individually, or CR followed by LF (CR+LF,
'\r\n', 0x0D0A)
This is also why you can retrieve the system-defined line separater from the System class as you did, instead of, for example, having it be some constant in the String class.
