Why play classic Pokémon now?
On the surface, the Pokémon games before the GBA era appear to be far too outdated to enjoy nowadays. Many of the commonplace features present in newer Pokémon games didn't exist in generations 1 and 2, such as natures and abilities. The graphics are 8bit and far less detailed than their modern counterparts. Numerous people find the likes of RBY (Red, Blue, and Yellow versions) and GSC (Gold, Silver, and Crystal versions) to be pointless to play so far out past their release. This brings into question - why would you want to play games so old? To answer this question, some context as to the history of Pokémon's development in the 90s is incredibly insightful.

The effects of this original design philosophy resound throughout RBY and GSC. The lacking of new, more complex mechanics such as natures and abilities in modern Pokémon games is made up for with gameplay that requires dedicated time and effort to reach into its deepest crevices. You have to go out of your way to optimize your Pokémon, to max all their stats in a time where EVs didn't exist, to plan and build their movesets, and to get every bit of customization on that Pokémon. Along the way, RBY and GSC, more than any other games in the series, give you the time and space to really get attached to your Pokémon and your individual story as a Pokémon trainer. GSC especially are much slower paced than modern Pokémon and expect you take the time to immerse yourself with a combination of daily events and tough-to-reach goals. It is simple enough for a child to pick up, but deep enough that you can spend hundreds of hours even after clearing the main story just picking away at optimization and teambuilding daily, whether it be to defeat Red, to clear the Crystal battle tower, or 100% complete both Stadium games on the N64. That isn't even to mention the level of customizability that opens up when game versions are connected to each other for trading and breeding!
Many people don't know that RBY and GSC have this level of depth. There's a variety of reasons for this, but one of the main ones is the Virtual Console releases of RBY and GSC on the 3DS eShop. Removed from their original contexts, not only were they not released alongside re-released Stadium games which essentially serve as dozens of hours of postgame for both generations, but they were released so far out from their original 1990s counterparts that many modern Pokémon players tried to play them in the same way you'd play a more modern Pokémon game, such as Pokémon Ultra Sun and Ultra Moon on the same console. RBY and GSC were never meant to be played in this way, and as a result the experience is lacking. However, if approached with the mindset of a fresh and different Pokémon experience, very unlike the ones since the GBA era, Pokémon's first and second generations can be some of the most fulfilling and in depth Pokémon experiences there is. Besides, even as just a look into Pokémon's origins and the mindset of the team in its history, it's worthwhile to play it casually just as a curious Pokémon fan.
1 https://www.polygon.com/interviews/2018/9/27/17909916/pokemon-red-blue-junichi-masuda-interview
2 https://www.nintendo.co.uk/Iwata-Asks/Iwata-Asks-Pokemon-HeartGold-Version-SoulSilver-Version/Iwata-Asks-Pokemon-HeartGold-Version-SoulSilver-Version/2-The-King-Of-Portable-Toys/2-The-King-Of-Portable-Toys-225900.html
3 http://lavacutcontent.com/satoshi-tajiri-ishihara-interview/
2 https://www.nintendo.co.uk/Iwata-Asks/Iwata-Asks-Pokemon-HeartGold-Version-SoulSilver-Version/Iwata-Asks-Pokemon-HeartGold-Version-SoulSilver-Version/2-The-King-Of-Portable-Toys/2-The-King-Of-Portable-Toys-225900.html
3 http://lavacutcontent.com/satoshi-tajiri-ishihara-interview/