Are U18, U21 and B-teams useless?

When I watch videos on player development, they all emphasize that first-team football is crucial for development. By that logic, U18 and U21 teams seem pretty useless—or maybe I’m just misunderstanding the point.

I also wonder if this applies to B-teams that are playable, like in my Getafe save where Getafe B has reached the second division of Spain. Does this count as first-team football?

Yes, it's about player development… and you're to develop a player in an academy before they can turn into professionals. However, this is like… you don't just throw everyone into universities, you do need them to study and develop, going through primary school and high school first. 

From every party's perspectives: 

Players: 

Players need to learn and develop and become decent enough, before they can gain match experience. For example, if you don't have the stamina and are out of breath after just 5 mins on the pitch, you're not going to learn anything in a match! The academy is here to prepare the kids for that… 

Yes, first-team games bring you the best match experience. The fact is that you can't have 40 players on the bench, and obviously, you can't make 20 substitutions in a game. Let alone how the result is… there's 90 minutes in a game, how do you share the playing time among all the players? 

So, the questions is: Let the youngsters sit at the stand and watch the senior members play every week, or Have them play against other players of the same level… which can benefit them more? 

Clubs: 

From a club's perspective, you do need to evaluate if any youngsters are good enough to make the first team. Sadly, not everyone can become first-team players, or are even good enough to play football in lower leagues. 

Yes, first-team football is crucial as you said… and as mentioned above, game time is limited, and I suppose you want to give it to your best young players, right? Having U21 and U18 may help you identify your best prospects, then you can allocate the best possible resources to them. This way, they can improve further and have a better chance to develop into star players.  

Besides, some leagues have restrictions on the number of players sending out on loan. So, you just need to find some game time for your young players within your club anyway… 

Management: 

Next, if you're in the management, do you prefer a first-team of 50 players OR 3 smaller teams which have different objectives and are managed by dedicated staff? For example, in U18, you may focus more on building their physical attributes, while you can start teaching tactical stuff in U21. 

How about training needs? A 16-year-old academy player, who comes to training after school and prepare for examination next week, can't possibly handle the same workload as a professional first-team player... 

Remind you that in the game, you can hire a limited number of coaches for your first team. 5 or 6 coaches working with 50 players would be a nightmare, and obviously not so efficient. On the other hand, U21 and U18 teams can recruit their own coaches. Same for physio and sports scientists...

Also, here is another benefit of U21 or U18 teams: If you happen to have first-team members recovering from injury, they can play at the reserves to build match fitness before being ready for the first team action. 

In summary, it's like education… universities are crucial, but if you want to develop great students and graduates, you don't just do things on the top level. It's important to build a solid foundation first (maybe this is the more common logic?), starting from the bottom, i.e. primary and high schools… 

So, I won't say these teams are useless… it's a matter of how you utilize and manage them. In fact, the academy is very much linked to the reputation of a club. Just have a look at the football world in real life…  

In FM's case, if you're to build a great football club, it starts from your academy. Without great academies, where do all those talents come from? OK, you may simply sign prospects from other teams (They must have great academies that can produce good young talents... a proof that it's not useless and worth investing resource on it.), but without building a decent academy, where do your train and develop them after signing from other clubs? 

One thing being important doesn't necessarily mean others aren't... especially the latter is such an important foundation. You won a game by 5:0, and praised the midfielders and attackers. How about the defenders? They probably played an important role by winning the ball many times for the team, and then resulted in those goals. They did well and deserved credits too… 

Well, by all means, you can experiment by ignoring the U18, U21 and others, and see how it goes… interested in how it turns out.