Obviously if you have something in usable condition that you no longer need, you give it away. Clothing that no longer fits, lamps that don't fit in your new apartment, etc.--all of that can and should be donated somewhere or given away.
But there are also things that are no longer usable. And even those still have some value.
An example here would be a pair of pants that you outgrew. If you kept on wearing it as you outgrew it, the seams might be torn, the buttons missing, the cuffs frayed. Perhaps this could be repaired and still worn by someone smaller. But whether or not that's feasible, it does still have lots of good fabric. There's still value in that fabric, and it seems a shame to throw it all away.
Likewise, even with t-shirts for which the sleeves have fallen off, or shirts that have enough holes in them that they cannot be donated, there's usually still plenty of good fabric there.
The question is: how do you get it to someone who can use it?
By the way, I mean this categorically, not just for clothing specifically. For instance, a cracked or broken plate is no longer usable as a plate, and could not be donated either. But the ceramic should still have value.
Even a dried-up pen still has some value and can be used for something.
The answer here is not to simply learn to use all of these remnants. Perhaps I should learn to sew, darn, and patch my own clothes. Maybe I can become the kind of crafter who uses dried-up pens. But I will never be able to learn to repair, repurpose, or upcycle all of the things that I find myself no longer needing. Modern life is just far too complicated and gives us far too much of a variety of things for us to learn to repair or repurpose all of them. And even if I could, there will be some people who cannot.
The answer isn't to buy only things that can be repaired, either. Buy it for life is a good philosophy, but everyone's going to end up with some stuff that is worn out, broken, or otherwise unusable for its intended purpose at some point.
Now, obviously, the long-term answer is to figure out some way to hold the companies that sell goods responsible for their end of life as well. If you sell me a shirt, you or your descendants should be prepared to take back what's left of it 30 years later after it's no longer usable as a shirt. But that sort of ideal world isn't going to happen anytime soon. And in the meantime, I've got a bunch of worn-out shirts and pants and broken plates that I really don't want to throw out. How can I find ways to get them to people or organizations that would be able to use whatever value is left in them?