I like to put a front cap on my camera lens when I do not shoot. Over the years, I have noticed that there are three types of caps on my cameras and lenses.
'Push-on' type: made of metal, plastic or rubber. My dad's old Zenit C has a metal one produced in late 1950s or earlier 1960s. In mid 1980s, he brought me my fist slr camera, Yashica Fx-3, from Hong Kong. It has a short-range zoom with a plastic front cap. This type of cap is simple, just a cover without any lock function, but no more common these days.
'Center-pinch' snap-on type: a common type today. See the Nikon one in the following image. Do you know why it's common? I believe, when you have a lens hood on your lens/zoom, you will figure out youself. Your fingers can easily reach the center-pinch to put on or take off the cap within a hood. Due to the structure on the caps, they often are thicker than the other two types.
'Clip-on' type: you need to press the two 'buttons' along the outside edge of cap when installing/taking off it. The structure on this type of cap is simpler, so they are thinner.