nl numbers lines; cat -n does too, with different defaults. The interesting use is not display but preserving order through operations that destroy it.
The sort-and-restore pattern
Number the lines, perform whatever reordering you need, then sort numerically on the prefix to return to the original sequence. This is how you apply a sort-based operation to data whose order is meaningful.
nl skips blank lines by default
nl numbers only non-empty lines unless told otherwise with -ba. If the line numbers are being used as positional identifiers, that default silently misaligns them against the original file. cat -n numbers everything.
Zero-padding for lexical sorts
If the numbered output will be sorted as text rather than numerically, 10 sorts before 9. Pad to a fixed width — nl -nrz -w6 — so lexical and numeric order agree. This matters most when the numbers become filenames, where the sort is almost always lexical.
Try it: Number Lines on SeoWolf's Notepad.