Having visited many second hand bookstores in different cities, there are absolutely books out there worth reading not available online- soke will never be available online. There are a lot of obscure books out there- old, self-published, or just really niche subjects- which will never be popular or wanted enough to find reproduced digitally.
Besides being an experience in and of themselves (used bookstores are often very unique and individual, sometimes operated by interesting people who do it out of a love of books or history), you can find some real gems and sometimes even terrible unbelievable books of decades past that would never be something you could find in a regular commercial/corporate book shop selling for profit and catering to whatever is on the bestseller list, what critics recommend, and what publishers want to sell.
Besides the experience and obscure books, used bookstores often are the best places to meet interesting people. Some people, myself included, love the atmisphere of these shops- the smell of old books, the piles of otherwise discarded tomes, shelves where you can find books that were once part of collections curated by bibliophiles- some of these books have annotations on them, some of them are on very niche subjects not easily found elsewhere, some so old if you look for information online you won't find anything. I've had and heard some really fascinating conversations in second hand bookshops I don't think I would ever have experienced otherwise. Some of these shops are a place of community, where people post business cards and event posters for things you wouldn't readily find out about easily- some host events in the store itself (discussion groups, games, book clubs, etc). Some have very comfortable atmosphere and welcome people coming to just hang out, no pressure to spend money. Additionally there is the possibility there for you to build a bond with the owners/keepers, who sometimes will bring out books they keep off the shelves just for you.
Some old book shops are absolutely packed with everything from textbooks to catalogues to literature- unsorted treasures waiting to be found. Books with no ISBN, no online presence whatsoever, items that cannot be found otherwise (unless you were looking for them specifically, and even then they would be really difficult to procure). There is a charm to be found in these shops that just cannot be replicated in common bookstores.