1. I can understand this command makes some space in the document, like empty lines. But what's the exact meaning of "space equivalent to the height of a 12pt font"? I can only understand the space equivalent to one line, two lines, or three lines...If you want to add blank space into your document use the \vspace{...}
command. This will add blank vertical space of a height specified in typographical
points (pt). For example, \vspace{12pt} will add space equivalent
to the height of a 12pt font.
2. When do you use this command in your work? Can you give me an example?
Many thanks in advance!