Significant? Or not insignificant?

Why do we sometimes use double negatives to describe objects, people or events?

“They made a significant impact on my life.”

“They made a not insignificant impact on my life.”

There seems to be a degradation in import in that second sentence. They should effectively mean the same thing. But of course they don’t. The first gives direct and obvious attribution of impact. The second does so but in a backwards, insulting yet complementing way. Are these different grades or levels of the same thing? Or are we uncomfortable in declaring baldly that something is definitively THIS, rather than sliding into a position that that something is *not not that*? Are we becoming wishy-washy in our assertions? Are we afraid of being incorrect in some stalwart position we might take for or against something or some ideal?

“Your actions sir, gave me great joy!”
“Your speech madam, filled me with loathing!”

Be strong in your convictions. State them directly, boldly.