What are the "variety of reasons" laptops make sense for the application? Mobility would be the obvious reason to infer - but do all of your computers need to be mobile?
Multiple reasons laptops are better than desktops for my homelab use case.
Most important reason is power consumption. When you have PG&E, this matters. Look up the rates if you aren't in our area, you likely don't understand this.
Sorta related to power consumption... laptops have a built in battery backup that goes for hours. Yes I can buy UPS's and mini PCs but that ends up more expensive, less efficient overall.
Also generally cheaper generic Amazon/etc mini PCs that would give me the same specs include trash-tier generic SSDs that simply don't last. If I buy an efficient small form factor desktop from a "legit" company like Lenovo, Dell, etc. they're MORE expensive than buying laptops. And I still have to have them on a UPS.
I can custom build PCs all day long... but they're going to be big and ineffecient. When you are already running a 1kw AI server, a workstation/gaming PC, and the rest of your homelab on a single 15A circuit, the less power your stuff uses the better. I can easily manage peak loads (not play games and run AI workloads at the same time, for example) but the constant load of various services I have running on my other systems needs to be as low as possible.
Or I can buy a laptop that sips power, has a built in battery, and includes a quality SSD from an OEM like Samsung or comparable.
And if/when my needs change, laptops are generally more useful for other things and are generally quicker to sell on the used market. Cheaper to ship them too!
At the end of the day, I'm not CPU-constrained on these systems, I'm not running them at full load all the time, and while it seems "wasteful" to pay for a screen that I only use for like 15 minutes to install Proxmox it ultimately makes a lot of sense.