How to use "i" of sed?

Shell Programming and Scripting
1

A manual page of "sed" (stream editor) describes "i" as follows.

[1addr]i\
text Write text to the standard output.

I do not understand the usage of "i" of "sed". Anyone, please show me some simple examples of "sed" with "i".

Thanks in advance.

2 
[house@discovery] cat test.file
this
[house@discovery] sed 's/this/that/' -i test.file
[house@discovery] cat test.file
that
3

i\ Insert text before a line.

$ cat file
Line1
Line2
Line3
Line4
Line5
Line6

# Add a line before the first line
$ sed "1i\\
> Add new line" file
Add new line
Line1
Line2
Line3
Line4
Line5
Line6
# Add a line before third line
$ sed "3i\\
> Add new line" file
Line1
Line2
Add new line
Line3
Line4
Line5
Line6