Diagonal lines are powerful tools for photographic compositions
Diagonals are a very powerful compositional tool.

Diagonals demand attention, this is why the warning signs on the road are triangles and diamonds. Even if they are only seen in peripheral vision they attract the eye. Diagonals create motion and irresistibly draw the eye to them; visually the diagonal line motivates the viewers eye to travel along it. If there are multiple diagonals the effect is multiplied.

Here the diagonals in the lower portion of the frame move the viewers eye to the flowers which leads the eye to the curtain where the orange panel leads to the top of the frame, where the paintings lead down to the red sofa where the horizontal line of the couch leads the eye back to the flower and around the image again and again.

The diagonal lines are helping to draw the eye to the purple flower - even when your eye is focused on the couch the effect of the diagonals on your peripheral vision is pronounced.

Use diagonal lines to move your viewers eye through your photograph along a predictable path and/or to draw attention to a particular feature of your subject.