To 'raise awareness' of weather events, as if one might not notice them otherwise. To be less cynical, people might pay more attention to weather warnings if they are named storms? Not sure how long that effect will stick if we have a named storm twice a month! I think you might not be far wrong saying it's about grabbing headlines.
In my home it definitely works. We say "Wow, a NAMED storm is coming tomorrow". We say it in a half joking tone, because the naming thing is a bit quirky, but we do pay more attention and are more careful when it's named.
