They're used to explicitly define less than and greater than symbols. If one wanted to type out <html>
and not have it be a tag in the HTML, one would use them. An alternate way is to wrap the <code>
element around code to not run into that.
They can also be used to present mathematical operators.
<!ENTITY lt CDATA "<" -- less-than sign, U+003C ISOnum -->
<!ENTITY gt CDATA ">" -- greater-than sign, U+003E ISOnum -->