(For anonymous. Thank you for your unending patience!)
Early Pokédexes were heavy, expensive gadgets which acted much like interactive encyclopedias, marketed predominantly to researchers and those with an interest in observing pokémon in their natural habitats. The initial models were manual, requiring the user to check off sighted species themselves, and boasted in-depth entries for all of the pokémon detailed. Such entries focused mostly on objective information: biology, migration patterns, habitat, diet, lifespan, etc.
After the establishment of Trainer Access Schemes, a second wave of similar programmes sprang up in several regions, offering simple ‘pocket dexes’ as one of their numerous benefits. These pokédexes were simpler, with entries centred more on the use of pokémon in battle, as well as tips on how to train, catch, and care for them. A flavour of the encyclopedic roots remained, but to a much lesser extent than before. Almost all early mass-produced dexes were regional.
In the modern day, dexes are a growing product, with multiple companies vying to create the most advanced models. Current Pokédexes, which are mostly national, feature photo recognition technology, mark species as seen on sight, and will automatically bring up entries and read them aloud where relevant (which gets pretty annoying sometimes). Use of the actual index is never reliant on internet access, as trainers often explore in rural and mountainous areas, but apps and web browsers are typically included as extra features. At this point, Pokédexes are effectively like smart phones. And, just like smart phones, they keep getting bigger - something that trainers continually complain about.
Unlike in the games, the act of completing the Pokédex solely pertains to seeing all of the pokémon native to your region, and this goal can take years. No trainer would attempt to capture every species they met, for both practical and moral reasons. Bonding with a wild pokémon to the point that they would willingly partner you takes time, and is not a process you’d go through for one you never intended to train. Moreover, removing a pokémon from the wild for bragging rights is pretty badly frowned upon. Not to mention that the cost would be astronomical - your PC Box subscription would go through the roof.
For most trainers, the pokédex is simply a guide. It offers tactical information, some useful background reading, and a bit of entertainment when you’re camped out in the Gym reception waiting for your (probably overdue) appointment. It’s also got a pretty solid GPS system, which everyone ends up relying on at some point.