Better control over it would be my guess, as well as disaster recovery.
Control over the OS, kernel and server source code would expose values you can tune to make it perform better under their specific workload (where as managed services tend to strike a balance between a wide range of workloads).
For disaster recovery, a previous client of mine got bitten by an RDS instance that was stuck in “modifying” state for 12+ hours (presumably until an AWS engineer manually fixed the problem). Being able to SSH into the machine as root would’ve saved us quite a bit of time (we ended up starting a new RDS and restoring from a - thankfully very recent - backup to get the service back online).
Control over the OS, kernel and server source code would expose values you can tune to make it perform better under their specific workload (where as managed services tend to strike a balance between a wide range of workloads).
For disaster recovery, a previous client of mine got bitten by an RDS instance that was stuck in “modifying” state for 12+ hours (presumably until an AWS engineer manually fixed the problem). Being able to SSH into the machine as root would’ve saved us quite a bit of time (we ended up starting a new RDS and restoring from a - thankfully very recent - backup to get the service back online).