Depends on how much/far you ride. A smartphone can more or less do much of what a dedicated bike computer can do to varying degrees of success but the battery life is better, doesn’t deplete your cell, and the computer will probably be more accurate when it comes to stuff like bike speed, trip tracking, etc.

Currently I just use my phone but there’s plenty of times Google thinks I’m driving a car when I’m actually riding my bicycle, ebike, or even a board sometimes.

Quite a lot:

• energy efficency: a phone with a screen on will depleate battery very fast. This thing claims 20 h battery lide while actually using it
• readability in full sunlight: most phones are not readable in full sunlight and consume a lot in this situation
• impermeability: while many phones nowadays can withstand some water, this thing is designed to be used under the rain

Is it worth it? I guess it depends on the use you make of you bike…

Adding to the comments already there: The image stabilization in your phone can get messed up if your phone is mounted to the handlebars for too long