Trade ins are an easy , hassle and stres free way to unload old gear. Some people are too busy or cant be bothered to try and spend the time dealing with a private sale.
Whether it's trading in a golf club or a car, you'll most often get much more if you choose to sell it privately but that can be a royal PITA so most people I know just trade their stuff in. Drop the car off, dont have to do anything , no cleaning no nothing, just leave it and get the $$$$.
One of my gigs is to offer a service to sell stuff for the people and take a commission above who don't want or cant be bothered. It can be a pain dealing with some folks but been doing it long enough to be good at the game. I've got a bunch of new in plastic Scotty putters up now and have them listed at > $200 of retail and get stupid offers and question, I just don't reply and block them. I've sold about 17 in the past 3 weeks. It used to be 90/10 good vs bad buyers but that's gone to about 60/40. To the point that I am upping my commission to 25% from 20.
A friend upgraded and so has his old 35 foot boat he was going to trade in but I will now be selling it once the better weather arrives. ....he works 12 hour days at a stressful job so has zero time to do that shit.
Plus with a trade , your payment is instantaneous. It can take months, even years to sell some stuff. I just sold a table that I had listed for 2 years and have 3500 sq feet of imported natural cut stone tiles that I have been trying to move for a year.