The newer Pokémon video games are often accused of being far easier than their older counterparts. The universal Exp. Share of Generation 6 and beyond takes much the blame, but this mostly just negates the chore of grinding. Recent Pokémon games' reusable TMs, on the other hand, take the planning out of teambuilding, granting unlimited access to powerful moves without any downsides.
TMs were, from Generations 1 through 4, single-use items used to teach a Pokémon a new move. In many cases, a particular TM had to be found somewhere out in the game world, with only one of that kind made available to the player for the entire game. This meant the decision to use each TM had to be made with caution; players had to figure out which Pokémon would most benefit from learning each TM's move, and they also had to be sure it wouldn't need room for a different move in the future, as overwriting the TM move would mean it was gone forever.
In Gen 5's Pokémon Black and White, TMs became infinitely reusable. Rather than constructing a well-rounded team with a potential answer to every challenge, players could now tailor-build each Pokémon's moveset to suit any situation, if they had enough compatible TMs on hand. Particularly useful TMs like Swords Dance and Substitute could also now be taught to multiple Pokémon, creating unlimited potential for easy sweeps through AI teams. The cost of purchasing TMs was significantly increased to reflect their infinite uses, but rarely has money been an issue in the Pokémon franchise. In Generation 7, the removal of HMs, previously used to navigate obstacles in the games' overworlds, meant team movesets had even more freedom to juggle TM moves.
Unlimited TMs Let Players Practically Rewrite Their Pokémon
Pokémon Sword and Shield's Generation 8 did create somewhat of a middle ground. Its new TRs function much like the original TMs in that they are single-use, but once a Pokémon has learned one of their moves, it essentially becomes a permanent part of that Pokémon's learnset. Should it be forgotten, it can be recalled at a Move Reminder for free, eliminating the risk original TMs carried of being wasted and lost forever. That said, Gen 8 still includes the multi-use TMs - TRs only limit the use of 99 former TM moves (plus one new move).
Apart from making the games easier, reusable TMs also encourage players to constantly adapt a handful of Pokémon instead of exploring the diversity of the series' roster or simply working around the strengths and weaknesses of each Pokémon. Players can just rewrite their teammates' movesets over and over to fit the battle at hand, and this takes away just a bit of the magic of a player connecting with their Pokémon.