The upper respiratory tract mucosal tissues are the places most respiratory viruses colonize first upon subject exposure. Intranasal vaccines can trigger mucosal immunity, especially secretory IgA antibodies, right at the site of infection. That’s something injected vaccines usually can’t do well.

Even if an injectable vaccine is able to prevent severe disease from those kinds of viruses, it is not completely "sterilizing" as the virus will still replicate on those mucosas and will be able to be spread even if in a less virulent fashion. Intranasal vaccines have the potential to stop those viruses before they have a chance to take hold.