The “first sale” doctrine (17 U.S.C. § 109(a)) gives the owners of copyrighted works the rights to sell, lend, or share their copies without having to obtain permission or pay fees. The copy becomes like any piece of physical property; you’ve purchased it, you own it. You cannot make copies and sell them—the copyright owner retains those rights. But the physical book is yours. First sale has long been important for libraries, as it allows them to lend books without legal hurdles. (Jenkins, Jennifer. 2014. "Last Sale? Libraries’ Rights In The Digital Age". College & Research Libraries News 75 (2): 69-75.)
Quite simply, first sale is what allows libraries to do what we do – lend books and materials to our patrons, the public.