If you mean a cheapish wired bike computer for basics of speed, trip distance, current time, etc...
- With wired computer no worries about cell phone getting damaged - rain, hitting a bump and falling off, etc. These things could happen with the bike computer to but it's $30 vs $500/$1,000.
- With wired computer screen is easier to see in sunlight. I have yet to have a cell that's 100% legible in bright summer sunlight, whereas the black and white led screens on cheap bike computers aren't negatively affected by bright light.
- Battery life. Cell phone often wouldn't last the entire ride without the battery running out during the ride or at least dying later in the day before I go to sleep. The small wired computer batteries last almost forever, I finally had one die after about a decade.

If I was looking for one that did gps I'd probably go through the extra work to use my cell...
- A better case would probably negate most concerns about damage etc, but it's more expensive and more hassle to use.
- If I searched specifically for a cell phone who's screen could be seen in daylight I could probably find one.
- I could add on an external battery for battery life, though that would introduce additional cost, complexity, and hassle.

Basically, it's cheaper and easier to use a less expensive wired computer as all I'm really looking for is speed, distance, and time.
I think the extra hassle would be worth it if I was using a gps for the whole route, but as I'm not doing that it's simpler, cheaper, and easier to just stick with a wired speedometer.