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This template is for rendering a specific character "<", not for actually performing math operations. For that, you want ParserFunctions.
This template can be used in place of a literal "<" when the latter would be interpreted as a syntactic marker. In particular, it can be used inside templates meant for subst:ing, together with the <includeonly>subst:</includeonly> trick, to allow the subst:ed output to contain HTML comments that contain other subst:ed templates, parser functions or variables. This works even under the new parser, which doesn't otherwise allow code within comments to be evaluated.
For example, the code "{{<includeonly>subst:</includeonly>lessthan}}!-- {{<includeonly>subst:</includeonly>SITENAME}} -->" in a template produces "<!-- Wikipedia -->" when subst:ed.