that's what people who love to rap and have mastered their craft do. As Raekwon said about Nas. Said he had 3 verses ready for Verbal Intercourse but they used the one we now hear.
Or those who rapped for a long time and kept alot of their rhymes?
DMX would do that all thru his career. In his book he xplained he didnt like to waste rhymes. Most of his shyt was older rhymes.