You misunderstand what -i means.
-i inserts a new line of text. It's meant for situations where you have a file:
line1
line2
line3
and you want to insert MYLINEXYZ in between line1 and line2.. you would use /write test.txt -i2 MYLINEXYZ to get:
line1
MYLINEXYZ
line2
line3
I assume you understand "insert" to mean "modify". That is not the case. Insert in this case means insert a new line, not to mosify an existing one. -l is used to modify existing lines (which is your version of "insert"). Finally, you need -a with -l because you need it to *append* the text to the end rather than replacing the whole line.
