Hi Reza- You could, of course, buy tubes of secondary colors, but I don't for two reasons: I don't want to carry around extra weight when I'm painting outside, and most importantly, when you mix secondaries from the primary colors that are on your palette, you get a color harmony that might not happen if you buy tube paints. To me, it just makes things easier to have fewer tube paints to fool with, but there's no right or wrong answer for which paints to have on your palette: whatever makes a good painting for you is the right choice!
