TWLoader has a higher resolution GUI, and can show animated icons from DSi-Enhanced games (SRLoader can do that too, even on a flashcard), although currently in TWLoader, games added from flashcard have them as still icons.
You can run games with TWL clock speed.
Why bother emulating DS when it's already got the hardware?
Because TWLoader hadn't become a successful project until shortly after this emulator port was made. At the time this had value but TWLoader blew it out of the water.
Why don'tcha buy a $5 flashcart instead?
Because people are finicky about what they spend their $5 on. In all seriousness though, TWLoader does have value in the fact that it cuts out the need for a flashcart and lets the 3DS do the work. For some 3DS owners, that may mean not having to tediously switch between a flashcart and a 3DS cart all the time.