Convenience for sure. In everyday training the bigger screen is really the biggest benefit.

In racing, it is really nice to have the bike computer sitting on the bike "autopaused" and ready for you to take off. How do you see your metrics from the forerunner during a race? Do you have the quick connect or whatever? Well if you have a dedicated bike computer you don't have to mess with any of that, or contort to read the watch on your wrist.

I can't imagine not having a dedicated bike computer.

Yeah, I have the quick release.
for racing I cant imagine why i would want a computer, its really not a hassle to hop on my bike and down the road a mile or two move the watch face from my wrist to atop my BTA.
Then again, I could have no idea what I am missing.

Other than convenience - nothing. I sold my bike computer and have been using my 920xt as sole computer for past 2 years. Zero regrets. Quick release kit let's me mount it to bike if wanted.

I am shopping for a bike computer simply for navigation. Tri watch won't do that. I've found a bunch of new routes and places to ride and don't want to print out directions or have to get my phone out.

But for me that's the only thing that would make me have 2 computers.

I'm in the same boat, and use my 910. My biggest pet peeve is waiting for it to sync with Garmin Express. My wife, and friends have dedicated Garmin head units, and besides maps, turn-by-turn cues, and live Strava segments, the biggest advantage is the sync time.

I stopped using the bike computer that was mounted on my handle bars. I just keep it on the bottom of my wrist so when I am riding I can glance at it if I need to. Not as easy to see all the time, but I try not to focus to much on things not the road.

From a noob perspective---I like having something to tell me speed so I can at least get some inclination of how I'm pacing. I use a Garmin Fenix--but also have a basic $30 Cateye wireless computer on the handle bar that's easy to read. That combo works pretty well for me.

To keep things simple, I only have a 910 XT.
Only one thing that can get out of order.
Only one thing for which I have to know how it works.
Only one thing for which I have to take care that the battery is charged.
Only one thing for biking and running.

I do not even mount it to my bike anymore. I just have it on the inside of my wrist, such that I can always have a glance at it when in aeroposition.

I have a cheap used Garmin 500 that I bought for $50. I dont upload anything from it and just use it to see the data better on the bike. My much newer Garmin watch is on my wrist and is easier to upload from with bluetooth later on.