Why is the "dengan" possible here? Is it necessary?
Bagus = something is good; dengan bagus = something done well (good) ... adding dengan changes an adjective into an adverbial phrase.
That's right. You could use "dengan baik" here as well
Thanks for this!
that singer sings good .. was not excepted .. I know it is not the usual statement but I feel it should pass
Sings good is not grammatically correct in English
It's because the adverbial phrase "sings well" correlates better. "Good" is just an adjective; "well" is the adverb.
Can dengan be exchange with yang here?
Almost an absolute no, but people would still understand that. "yang" doesn't convey the meaning of process as good as "dengan"
as slang it can be said as: Penyanyi itu nyanyinya bagus