Connecting to ANT+ spin bike

No one here can guess what kind of capability your spin bike has in this regard. Refer to the manual for the spin bike, or at least tell us the name and model of the bike.

I must be missing something, because it seems like a pretty useless feature if all it does is display my HR on a monitor...

Yes, you are missing something. Seeing your heart rate during a workout is hardly a useless feature. Having a spin bike capable of receiving ANT+ heart rate data allows you to go for a workout with just a chest strap, for example.

If it's an older spin bike it's possible it supports the ANT+ fitness equipment profile that has mostly died off in newer Garmin devices. It was actually pretty cool since it supported pairing using proximity but it wasn't very widely adopted. Here's an old page with information on that: https://www8.garmin.com/intosports/antplus.html

If it's a newer bike you may be able to pair it as a power meter and/or speed and cadence sensor. You would need to add them as sensors to your watch. If it's in a spin studio with multiple bikes you will want to find the ANT+ ID number to make it easier to know which to pair with. You might be able to find this in the menu on the bike console or possibly printed on a label on the bike somewhere.

It might also just simply be for receiving ANT+ HR data. Like tmk2 said it's not possible to know the exact capabilities of the bike but here are some possibilities.

Most Spin bikes are beasts from the dark ages.  A lot of Gym equipment uses a proprietary Polar ANT comms to read HR and display on the equipment display and these will not read Garmin ANT HR straps.

I have a Spin bike at home that I use a great deal and actually threw away electronics they were so useless.

However, I currently use a Wahoo HR strap and Wahoo Cadence and Wahoo Speed sensors - one on each shoe.  I can feed these into my Fenix 5, but there is little to gain over just mounting a phone on the Spin bike and using the Wahoo App which is really good.  Whilst the Fenix 5 can capture Spin bike data, it is nowhere near as good as a good phone app.