Every team carries players who are not on the active roster but are attached to the organisation. The arrangement solves two different problems at once.
The active roster is smaller than the workload
Only a limited number of players may be active on game day, and the roster is built to cover positions rather than to supply bodies for training.
Full-speed practice requires more players than a team is permitted to dress, particularly for units that need eleven on each side simultaneously.
The developmental roster fills that gap, providing the numbers without occupying the places that count towards the active limit.
Scout team work is a specific role
Preparing for an opponent requires running that opponent's plays against the starters during the week.
Players are selected partly on how closely they resemble a coming opponent's personnel, so a squad member's value can depend on who the team plays next.
This is genuine preparation rather than filler, and a scout team that reproduces an opponent accurately is a meaningful contributor to the result.
It functions as an injury reserve
Injuries occur constantly, and a replacement who already knows the terminology and the scheme is more useful than a better player signed off the street.
Elevating a squad player for a game is permitted a limited number of times before he must be signed to the active roster, which forces a real decision.
Teams therefore keep players at positions where they expect attrition, which is why the composition shifts through a season.
Players are not exclusively held
Another team may sign a developmental player to its active roster at any time, and the original team cannot block it.
That rule prevents teams from stockpiling talent cheaply and gives players a route upward that does not depend on their current employer's plans.
The counterbalance is that the signing team must keep the player active for a period, which discourages purely opportunistic claims.
Why it matters for developing talent
Positions requiring the most learning benefit most, because a year of practice repetitions in a system builds knowledge that no amount of individual training provides.
A substantial number of eventual starters spent time on a developmental roster, which is why teams treat the group as an investment rather than as spare parts.
Pay and protections for these players have been a recurring subject in labour negotiations, since the role is full-time work at a fraction of active-roster terms.