It helps you to have better view on your players. Also, when you are selecting player for the lineup on the lineup page, you have "first team" and "reserves" separated which helps you to do it faster and easier.
Just be careful not to click on "release player" button, instead of "reserve team". There is also 2nd level of protection when confirmation button appears if you decide to release your player with a red button "yes" to confirm :)
suggestion:
that when you transfer a player to the reserve team, he cannot be there for less than 5 matches and that there is a possibility (let's say 30%) that he will get higher grades in training
predlog:
da kada prebaciš igrača u rezervni sastav, on nemože biti tamo manje od 5 meča i da postoji mogućnost ( recimo 30% ) da dobije veće ocene na treningu
@Panta - There will be noticeable additions regarding that area (maybe not in a way mentioned above), but also, two upcoming updates are reserved for some other features 😀