The entire concept of the Pokédex makes no sense ?!

Okay this thought just went into my mind randomly but… the Pokédex doesn’t really make any sense ?! It’s a portable encyclopedia that registers every Pokémon and creates an entry to it. And your mission as a trainer sent by the Pokémon professor is to fill it up ! So he can have this encyclopedia! But that doesn’t make any sense… the entrys already have had to be written once already ?! Since there is so much information in them, that wouldn’t normally be found out by catching/encountering the Pokémon once ?! That means that there already is information on every Pokémon ON THE DEVICE, that reveals itself after encountering the Pokémon ?! Why do we then even get sent onto the journey, if all the info needed is already on the thing? Or am I just getting the entire concept of the Pokédex wrong ?