Keep in mind that engineering is managing compromises.

The Garmin 800/810 are small form-factor GPS units with navigation. They work like car and smartphone navigation systems.

They also provide display cycling-specific data cadence/hr/power (smart phones can display this but it's a bit more effort). The smaller size makes it easier to mount on a bicycle but the display is small. The Garmins have much better battery life than smartphones generally and better weather resistance (you'd need some case/cover for the smart phone).