Helps keep air out. Sealed scope with vacuum inside would work well too but would take heavier construction and more exotic seals. Filling with nitrogen insures no moisture inside (fog prevention) and nothing to corrode the innards.
Nitrogen fill also lets manufacturer know if unauthorized tampering with the innards has been done when a scope is sent in for warrenty.
A scope without any gas inside (i.e., a vacuum) would be subject to having air forced in by atmospheric pressure (some might say "sucked in by the vacuum"). Filling the scope with nitrogen under a greater pressure than atmospheric pressure keeps air out. Nitrogen, of course, is totally inert.
