Dealerships usually work by focusing on what you want. If you want a low monthly payment, you'll get it... in exchange for a shitty 6-year loan so you pay $35k for a $25k car. If you want a 0% interest rate often they'll blow smoke up your ass about your amazing credit score (the best they've ever seen!) then charge you MSRP for a car nobody pays MSRP on, hoping you're so pleased with the 0% rate that you don't notice.
With these trade-in offers, they'll try to make you really happy for getting $5k on your junker, but that $5k is contingent on taking a crappy deal on the new car. So you make $5k on the trade-in but lose it right back with a too-high purchase price or crappy loan, or extended warranty, etc.