I am always confounded when I get off a flight at a vacation destination and see people dragging these 80L roller bags around. What are you putting in there?
Pack the absolute minimum. If you really need something, you can almost always buy it wherever you are going. Even trekking in the deep Himalayas, there was always a spot to buy an extra t-shirt or socks every day or so.
If I'm going scuba diving at a vacation destination then I need a large roller bag just for dive gear, plus another carry-on bag for my camera rig. There's no way I'm going to dive with shitty rental gear (except for simple, heavy stuff like tanks and weight belt).
Back when you could bring the more fun stuff back from Europe (meat, cheese etc) my big suitcase would be well over half empty on the way out. Quite a bit heavier on the way back ;)
On the return from my first trip to South Africa I carried 12 bottles of wine in my luggage.
That was back when flights included two free checked items.
On my second trip to Europe one of my suitcases was full of T-shirt swag to give out at a conference. Lugging both up the stairs, across the train tracks, and back down was a hassle.
Things were a lot looser then. I brought a 6-pack of Negra Modelo as carry-on for a trip to Europe. The airport x-ray staff in Albuquerque recognized it on the screen, which impressed me. They had no problem with it.
We travel out of a single roller bag for a family of 5. Plus we each have a backpack with miscellaneous stuff. We generally leave a little space in the luggage fo whatever we might pick up during our stay. Our kids are 6, 6, and 4 and we're currently on day 8 of 11 in Hawaii. It's been fine, we just do laundry every 2-3 nights.
And throw it all away before you fly home? That's like saying "Why wash dishes when you can just throw them away and buy new ones"?
As an aside, you should absolutely wash your clothes after buying them before wearing them. They're covered in chemicals that aren't great for your skin. I usually pack a week's worth of clothes and do laundry once a week while traveling. Doing laundry more often is a drag.
Why waste time and energy on chores such as laundry and shopping, if you can avoid it by taking a bit more stuff with you? A big roller bag is almost as effortless as a small one, as long as you are going to a semi-civilized place.
My favorite way of traveling involves a 20l backpack and a small checked bag. There is usually enough space for whatever I need, I don't have to worry about overhead space, and I don't need special travel versions of the things I use. But sometimes I take a larger (~100l) suitcase, or even both for longer trips with multiple activities.
As for trekking in the Himalayas, you should always carry clothes you don't expect to need and gear you hope you won't need. Mountains are objectively dangerous, and bad things do happen occasionally. Such as the unexpected blizzard on the eastern side of Everest last weekend.
Lots of kids' clothes. Kids' toys. Kids' books. Kids' medical kit.
Clothes that span multiple seasons or climatic zones: warm, cold, heavy rain, snow. Extra shoes. Especially when you know that your things will get wet and that you will have only occasional opportunities to dry them.
Suit/dress/fancy shoes if going to a wedding or other formal event.
Did a motorbike trip in Vietnam with my parents and they rolled up to the airport with 4 large (soft) bags. I was with a backpack. Guess who was the porter throughout the trip.
Pack the absolute minimum. If you really need something, you can almost always buy it wherever you are going. Even trekking in the deep Himalayas, there was always a spot to buy an extra t-shirt or socks every day or so.