Other than giving you backstory to many things going on in Destiny, some of these cards can do something specific. For killing enemies on the separate planets, once you get the grimoire card 2 or 3 for that planet, you will be able to farm 2 of every upgrade material (i.e. spinmetal, spirit bloom, helium filaments, and relic iron). I believe that getting the specific kills upgrade will allow you to get more glimmer from majors of that class.