Could anyone please tell me how these characters mean "Of course!"/"That goes without saying!" when placed together?
Because when it's translated one by one, the characters don't seem to correlate to one another at all.
那 : that
还 : still
用 : use
说 : speak
I'm just curious though.
The point is 用 doesn't mean "use", but "need" here.
（多用于否定: 需要） need:
There's no need to turn on the light. 不用开灯。 I don't think you need to worry in the least. 我看你一点也不用着急。
And 还 is used for emphasis,
（用以加强语气）:
That goes without saying. 那还用说! This is the limit!; This is simply atrocious! 这还了得!
那还用说 is a rhetorical question (echo question), it literally means ”need to say that?!", i.e. "no need to say", then "of course" or "that goes without saying".
那还用说！ means "not even deserves to mention it", implying something is very obvious and no need to address it or put it on the table.
Stretch a bit:
那：that thing
还用（equal to 不用）：not necessary to
说：speak, mention
