Mikhail Botvinnik was a big supporter of training games as a tool for the chess improvement. He himself played a number of such games and later published some in the collection of his games. He showed how ideas developed in training games helped him to win the competitive games. In fact, Jan Timman published an entire book dedicated to Botvinnik’s training games.
As for myself, about 10 years ago I played a match of 10 training games against an opponent of roughly my strength. I lost the match by 1 point and overall it was a good experience. Both me and my opponent noticed that we were willing to take more risks than in usual tournament games. It also helped my opening repertoire to include the Open Sicilian.
Here is my analysis of one of Botvinnik’s training games: Part 1