If KDE/Gnome applications work in Gnome/KDE then...

[pzkfw]

Why did they bother coding two applications for the same thing? Like Kedit and Gedit. What's the point.
I can only assume they used to not work in the other DE.

[Zorael]

They are part of each respective desktop project. Gedit is part of the GNOME suite and Kedit (now KWrite) is part of KDE. As such they're based on different window frameworks (GTK and Qt4), so while they do work regardless of what environment you're running, they "fit" better into its own. There's really nothing more to it.

They aren't necessarily written by the same people, so "why did they bother coding two applications for the same thing" doesn't really apply. It's more a case of "why did different people code separate applications that provide the same functionality". Why are there different models of cars?

Much like there are different Windows media players with different ideals and design decisions behind them, there are unix programs that provide the same functionality as others. Then it's up to you to decide which you like best - much like many opt to not use Windows Media Player and instead use VLC or Media Player Classic. Perhaps one fits your environment (GNOME, KDE, Xfce, ...) better than the next, perhaps you're short on system resources and one is more lightweight than the other, perhaps you just like the features and design of one in particular.