Playing friendlies is of course a valid option, but it's not a "training" option, in that you, as a manager, should be very well able to tell your boys "OK, you, you you, line up in central midfield and have a go for about 20 minutes. If that doesn't seem to be working out, I'll have you play wider for another 15, come back and we'll discuss it." And so on.
ie, these "training matches" allows the user (you) complete freedom to mix and match players as long as their fitness (and your patience) allows.
Friendlies can only be arranged once a day, and the usual restrictions like substitutions also come into play.