Titanium white is very opaque while zinc white is very transparent. You can see this on the tubes in the image. The three black lines at the top of the tube are there to show you the transparency of the paint. The more of the lines that you can see, the more transparent the paint. This makes a difference when you mix colours.
For example , mixing titanium white with red will give you a pastel pink colour.
Whereas mixing zinc white with red will give you a lighter shade of that red colour because of the transparency of the zinc white