Long strings can be used for storing binary data. Because the length of the string is not determined by searching for char #0 (it is stored outside of the string), you can use all binary data - even #0.
These long strings are ideal for binary buffers, because they are automatically allocated and deallocated from memory. By using long strings as binary buffers you can create memory-leak-free applications!
However, you have to be careful when writing your strings to the display because most graphical controls cannot display binary strings (they expect null-terminated strings).
Of course, you cannot cast this binary string to a (null-terminated) PChar.