New furniture has one job, to look like new furniture. It arrives flawless, smells like a factory, and announces that it was purchased recently and probably from the same place as everyone else's. There is nothing wrong with new. But the rooms that stop you have almost always got something old in them, and that something was usually owned by a stranger first.
Secondhand furniture carries something new pieces cannot fake. Age, patina, the small marks of a previous life. A wooden table that has been used for thirty years has a depth in the surface that no finish out of a factory can replicate. It looks like time, because it is time.
The best piece in your room is probably one a stranger gave up on.
There is also the matter of quality, and this is where it gets practical. A lot of older furniture was simply built better. Solid wood instead of veneer over particleboard. Joinery that holds instead of glue that fails after two house moves. You can often buy a genuinely better made piece secondhand for less than a worse one new, which is the rare situation where the cheaper option is also the superior one. The factory has spent fifty years making furniture lighter, faster, and worse, and the proof is sitting in every estate sale in the country.
The catch is that thrifting is a hunt, not a transaction. You do not go out and buy the perfect sideboard on a Tuesday afternoon. You look, you wait, you walk away from things that are nearly right, and eventually the right piece appears when you were not specifically looking for it. This frustrates people who want a finished room by the weekend. It rewards people who are willing to let a room come together slowly.
A good piece can be reupholstered and refinished. Good bones are the one thing you cannot add later.
Estate sales, marketplace listings, auction houses, the back of a charity shop, a relative clearing out a house. The sources are everywhere and most of them are cheap. The skill is in the eye, learning to see past a bad finish or ugly upholstery to the bones of a good piece. A solid frame can be reupholstered. A dated finish can be sanded back and redone. A scratched surface is often just a scratched surface on top of something excellent. What you cannot fix is bad construction, so you learn to read the joinery, check the weight, and ignore the surface drama.
Mix one or two found pieces into a room of newer things and the whole space gets more interesting immediately. The contrast does the work. The old piece makes the new ones look intentional, and the new ones make the old piece look like a deliberate choice rather than a hand me down. A room that is entirely new looks bought. A room with one good old thing in it looks lived in, which is the quality everyone is actually chasing and almost nobody can buy off a showroom floor.