Connecting your shapes in a painting is very important for a painting with good sense of depth and atmosphere. Although the world around us is filled with details, we only see them when our eyes are focusing on one thing. When your eye focus on a specific thing, other things in your peripheral vision will become blurry and they blended together. That's the effect we want to achieve in our watercolor painting. By connecting shapes and let them blend into each other, you essentially push them back in your painting. Which will allow the main focus (figures, cars, houses..etc) to pop out more.
If you look at some of my landscape paintings, you will see many of them have very simple background. I need to merge numerous skyscrapers in a busy city street scene into 2~3 shapes. I painted a bunch of retail stores with one big shape in the background. And when I paint a forest of trees, I make hundreds of individual trees and leaves into one big organic shape.
All of the above paintings have one thing in common - They all have simplified shape background. So when you look at those paintings, your eyes will naturally focus on the figures or the cars. That's what makes a painting "feel" realistic. The paintings feel real not by how well you can paint details, but how well you can mimic how our eyes perceive a scenery. Next time when you paint a scenery, ask yourself this:
Are these the focus of the painting? If not, simplify them by connect and combine them.
It is a lot more challenging than it sounds. Because when our eyes looking at the reference material while painting. We starting to see every little detail from the source, that's because our eyes can only focus on one area at a time. It is important to constantly asking yourself the above question so that you don't get lost in detail.