Since Pokémon is a playable game most of all Pokemon cards are legal to use in decks but this does not speak for all Pokemon cards since some of them have eventually been banned from decks because they are too strong and have abilities that give users of them an unfair advantage. It never hurts to know whether a card is banned from a tournament before you end up going so you are not disappointed when you are unable to play with that card and must remove it from a deck. That actually happened to me once before as well. I was with friends going to the Pokemon center in New York City. The deck I was using relied mostly on the Pokémon fortress with a special Pokemon power that whenever Pokemon switched from being active to the bench they would take 10 damage. The Pokémon power was spike. The problem with this card was they banned it from the tournaments because of how nasty of an effect it was. And the strategy of play was really easy since you could set up 4 on your bench and then anything switching would take 40 damage just for switching. That is also why gust of wind was changed so that the opponent was to pick one of his Pokemon that gets switched. So when I got there I couldn't participate because my deck had been banned from usage during the tournament which was pretty depressing.
So besides having an all Pokemon cards list you would also want to make sure you have all of the most updated information about what is legal for using in various tournaments. I can't say if they still hold as many tournaments as they used to at conventions or at the Pokémon centers since I know they have the Pokémon online play as well. And it has been quite a number of years since I played back in 2003.
Pokemon cards can still be a good investment in the future since many of the original base set Pokémon cards are very valuable now especially the first edition ones that are in really premium condition. As more Pokemon cards get older they too will see an increase in value as they get harder to find making them valuable to collectors. Having lists of all Pokemon cards is a good guide to a collector that doesn't know enough about what cards each set actually had. I also provide information on error cards since many error cards are still considered part of the set and it makes it even more rare if you have a completed set with all the error cards.