Back in the days when books were made with handmade paper, deckled edges were regarded as an imperfection in the paper and bookmaking process, and all edges were cut cleanly.
Around the late 1800s, though, books with rough edges became popular. People appreciated the look and feel of deckled edges, and as a result, the pages of many books were left untrimmed.
Nowadays, our manufactured paper is made with clean edges automatically. BEcause it is processed that way, we have to take extra steps to rough up the edges of a book’s pages.
It’s funny to think that before the 1800s, books were altered to fix the imperfection of rough edges, and these days books are altered to achieve that imperfection to get that classic look!