You can understand why background is important by looking at the picture below.
A few days ago I draw the first picture without background.
But the picture did not seem complete to me.
Many artists like me put more emphasis on the painting front.But if you notice a little you can see.
The portrait looks more decorative because of the background of my painting.Due to the use of background dark color the portrait is coming up to the front.
software: clip studio paint.