Allow me to show you the difference courtesy of Websters dictionary online:
usefulness
Main Entry:
use·ful·ness
Function:
noun
Date:
1617
: the quality of having utility and especially practical worth or applicability
HOWEVER
uselessness
One entry found.
useless
Main Entry:
use·less Listen to the pronunciation of useless
Pronunciation:
\ˈyüs-ləs\
Function:
adjective
Date:
1592
: having or being of no use: a: ineffectual <a useless attempt> b: not able to give service or aid : inept
— use·less·ly adverb
— use·less·ness noun
Hope this is of help you you
