Personal knowledge gain and high levels of satisfaction. I’m 50% with OP on subject, as it does mean that lots of carbon copies can exist because chances are thousands of people have the same planks of wood and trees as you. Personally however, I prefer to do everything myself or within my team. It’s much more gratifying making your own assets, you have full control over the appearance and behaviour of your game and most importantly: you learn while doing it. Spitting out rapid prototypes is all well and good but if it’s not teaching you anything new then you’re probably going the wrong way about it.
For those that just want to get their ideas out there and don’t strive for personal development, marketplaces are great. On the other hand, you run the risk of not looking like you have the skills to pull off the idea on your own. I’ve seen and played a lot of games that have default UDK assets (I still laugh manically when I watch TotalBiscuits video of Garry’s Incident and it still has the default UDK sky and foliage). That studio shot themselves in the foot anyway when they we’re exposed for all the shady things they were doing, but just by seeing the game I felt as if the studio was lazy.
In my opinion (and it is just an opinion so pls don’t flame me), using Marketplace/mass-available-content runs the risk of making you look both unprofessional and unskilled. That said, it’s still a great resource. I only plan on buying content I can learn things from, because that’s my main goal.