The answer can be summed up in one word, "Cohesiveness". Painters usually want the elements of their pictures to fit together, and color is a too powerful a tool to overlook, whatever your goal happens to be.
In the watercolor below, Doorway to the Palazzo Barbaro, by Anders Zorn, Only three colors have been used: black raw sienna, and permanent red. The bluish tints are simply the black thinned with water. The painting seems to portray an integrated light phenomenon and a believable space.
Now, take a look at this landscape:
Alright, I know I'm overstating the case, but I want you to see what the danger is in being a profligate colorist.
For homework, choose a set of primaries that appeals to you; one only of red, yellow and blue. Make a simple version of a color photo or a live scene, in which all the colors are made by mixing the three primaries you chose. here are a couple of potential images:
There will; be compromises in this process - some colors will not be possible to duplicate - but have faith. The result will be an overall cohesiveness that cannot be achieved with an extended palette. Have fun.