In short a high quality product fit for purpose :-)
Better casework, better heatsinks, better connectors, better protective devices, better pcbs, better quality/specification components, better design, better quality output, better power factor, better electrical and physical noise performance, better EMC performance................ :-)
An install dimmer should be a physical package designed to fit with industry standard cable/cable management techniques and common layouts, you should expect good tech support, good spares backup, comprehensive testing and specification, robustness, long term reliability.... the list goes on.
Simon