Where is need and where is not need width 100%; on basic elements like html body

Some themes I saw:

html, body {
	width: 100%;

But what about height: 100%;? And above example what means for body need to set width: 100%; to get grid work? I think not but on html? Why they have set this in templates? And where need to set height: 100%; in body or html tags? or not need? and above example is also not need to set?


Please clarify the question a bit

Basicly the width 100% is needfor for a basic css markup or not? Because sometimes have set but why? I understand that for height and body height: 100%; is useful when want a full screen image to set and that way apply max-height: 100% to photo container is something like height: 100vh; So the question when need to set it? What situation?