If you recently matched to Marriott Platinum due to the status match with Starwood from the Marriott - Starwood merger, then one of the new perks you get is MileagePlus Silver status on United, through the RewardsPlus program.
The main limited benefit of Silver status is free Economy Plus seating, which isn't available even to other Star Alliance Gold+ members.
- Priority Check in
- Priority Security
- Upgrades, behind all other elites
- Reduced change fees
- Extra checked baggage