To be fair it would be pretty useless on a Class 350 as it's a commuter train designed to make multiple stops in relatively short distances.
I believe only the 377s had speed sets, but it was disabled entirely because drivers were slowing down using the speed set instead of the brakes.
I would say it's more about comfort. I've read somewhere that drivers can be more focused at watching the signalisation when they don't have to constantly monitor the speed of the train.
However, not having to concentrate on speed means that the driver may concentrate less overall. Some railway staff I've spoken to have had real concerns with the 80xs which have been limited to 124.9mph, as it increases the potential for overspeed incidents should the limiter stop working. It's ironic that it was brought in to prevent the incident where a TPE 802 was driven at 140mph.