This is a great post on the basics of training and why every manager should be looking to invest time and money in training. For me it is the most enjoyable part of the game; investing years into seeing your players develop to push your team to promotions and league titles, represent the U21 and NT and return some of that cash you've invested in merchandise through NT representation and presence on leaderboards is what makes it all worth it.
One thing I think that is worth adding to the training section is how investing in the Gym infrastructure can also boost training. If anyone isn't aware, a Gym can be built under the 'Team Infrastructure' section of the Arena page to further enhance training, depending on what level of Gym is built. There are three levels of gym ranging from Level 1 (cost of £200k to build) to Level 3 (cost of £1M to build). Depending on what level of gym you have depends on the number of 'cross-training' slots that are available for your trainees, so level 1 has 1 cross training slot of 10% and level 3 has 3 cross training slots of 10%.
Note: it is worth noting only trainees who have qualified for training receive this cross-training boost.