why we use horizontal line?

Answer 1

To give seperation
you can use lots of codes to design ur own line of separation if u want

Answer 2

From W3C website:
"Changes in HTML5
Although previous versions of HTML defined the hr element only in presentational terms, the element has now been given the specific semantic purpose of representing a “paragraph-level thematic break”.
Details
Some examples of thematic breaks that can be marked up using the hr element include a scene change in a story, or a transition to another topic within a section of a reference book."

Answer 3

to specify that we are now done with a certain topic

Answer 4

to design the webpage uniquely

Answer 5

hr- horizontal rule
own line of separation if u want