Painting is about edges
This is where two colors meet. When I was a kid I used to do the paint by number painting kits. When you do these each shape has a number so you just paint inside the numbered shapes. When you change to another number you are changing to a different color. Where these two colors meet is the edge.
After I started to do oil painting on my own with a blank canvas I still tried to paint like I did when I did paint by number. But I learned there are two kinds of edges; hard and soft edges. Most of the time you paint with soft edges (blurred and softly blending the two colors together)The hard edges are around the center of interest.
To illustrate this in the graphic below of the painting "A Cuppa and some Flowers" you can see all the hard edges around the flowers and the soft edges around everything else. The flowers are the focus so that is where the "hard" edges are located.
Notice the PINK edge against the red flower behind it. The yellow flowers just POP out at you with the color contrast! This is exciting to an artist.
Blurring the edges or making them "soft" helps to de-emphasize that area and force your viewer to look at the hard edges. This is where the center of interest or focus is.
When people look at a painting, they need to have one thing to focus on or the center of interest. The painting should not be cluttered with too many things going on or they will get confused about what the painting is about. More information on this topic of composition is found in the Oil painting basics page.