As soon as you start having you own server/network rooms, yes.

You will have to monitor your HVAC systems, your fire extinguisher systems, your alarm systems (for smoke and water leak detection), your electrical systems... You will need to make decisions about which type of system to install. You will have to set up maintenance and repair of these systems and interact with the specialized technicians who come to service them.

Not saying you have to become an HVAC expert, but you will definitely learn some stuff outside of computing/networking.

Yes.

At a basic level, most small companies still need a firewall/router and some switches in a closet. Maybe a local backup server? And a UPS to keep it going, and now you need to worry about the temperature.

Do you own the building? Probably not. Does the building landlord keep the HVAC going on nights and weekends? Probably only at a level sufficient to get things back to normal around 8AM the next work-day. So even if you install your own A/C: do you have sufficient power? Where does the exhaust go? Do you have a drain for the water you take out of the air, or a bucket? A nice bucket full of slightly oily water next to your machines? Better make sure there's nothing on the floor.

Add a few temperature sensors and flood-detectors, too. Rig an alert system.

At some level, sysadmins are at the boundary between hardware and software, between the real physical world and the abstract manipulation of symbols and data. With that in mind, you can ignore HVAC to the extent that "facilities" understands your needs and keeps them in mind during routine operations over a year, a day, or a month, from their project managers down to whoever is on site that day perhaps testing generators, switching away from the chilled water loop now that it is winter, and so on.

As a programmer who relies on these services, if the server room "feels hot" but I haven't gotten anything out of monitoring, I will stick my head in. Your monitoring and your A/C can fail at the same time and I have had that happen more than once.

Yes, because the ordinary HVAC service runs on a "best effort" basis where some downtime or reduced capacity is acceptable and you can try and fix stuff if someone complains; but for a datacenter you suddenly need 24/7 99%+ uptime with rapid response in case of failure, because losing AC is only just a bit worse than losing power, and it's easier to have redundant power than truly redundant AC.

datacenters are competitive HVAC implementations with computers inside. its all hyper-abstracted in the cloud for most people now, but anyone who dealt with on-prem equipment pre-cloud knew their buildings facilities and maintenance team well (as well as basic cfm, btu, and kva numbers).

I happen to have a friend in HVAC. I bought some gauges and basic tools and he trained me on troubleshooting the AC, its really helpful.

Having redundancy or at least a backup plan is better. You have to consider all of the risks and what is at stake and how much downtime you can handle.