What was the importance of First Day Cover? Answer with reference to the chapter 'The Third Level' from the book 'Vistas'

Dear Student,

First-day covers are envelopes that philatelists mail to themselves with newly issued stamps affixed on them. The envelopes are mailed on the very day the stamps are released. The postmarks indicate the date on which the envelope was mailed and thus, shows the date of issue of the stamp. Blank papers are put inside the envelopes, and generally, they lie unopened in the collector's desk. In the chapter, Charlie finds a first-day cover addressed to his grandfather's Galesburg home dated July 18, 1894. Perplexed at the presence of the letter, he opens it and finds out that Sam has reached Galesburg.