What is "applessdstatistics" doing on my non-SSD, non-Fusion drive iMac? I have a mid-2010 iMac that has regular HDD (not a Fusion drive, just good old spinning disks). Recently, after reboot, I was hearing intensive HDD I/O sounds and firing up Activity Monitor I noticed applessdstatistics process was reading a lot of data.

I found a way to disable it via Terminal command:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.applessdstatistics.plist


But the question is: what was the process doing on my computer in the first place?

Does it do any tasks that would—nevermind the process name—benefit a regular HDD? Meaning: should I re-enable it?

I'm on up-to-date macOS Sierra 10.12.3
For whatever reason applessdstatistics started appearing for many non-SSD users after the macOS Sierra 10.12.2 update. Apple is aware of the issue.

As you've discovered, if you do not have an SSD it is totally safe to disable applessdstatistics with:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.applessdstatistics.plist


And no, there is no need for you to re-enable it.

[EDIT]

After reading the comments I thought I should add some additional info about this issue.

Based on the advice provided by Apple on 8 February 2017, there are actually two issues that appeared around the same time concerning applessdstatistics. I describe these below:


The first issue is the one directly referred to in the OP's question. This is the issue of Macs with only hard drives suddenly having applessdstatistics appear and start reading data from the hard drive (typically in 4GB lots). applessdstatistics is only supposed to run on Macs with a SSD installed, not on Macs with traditional hard drives installed. This issue has affected a large number of desktop users with internal hard drives, primarily older iMacs (although this may only be because there are more iMacs in the field compared to Mac Pros and Mac Minis). I am unaware as to whether it is affecting MacBook models with only hard drives installed (as Little Eden implies in their comment below), but I am aware of the following issue (see 2 below) which could be what Little Eden is referring to.
The second issue, however, does affect various MacBooks (and other Macs) which do have a SSD installed. The issue is not the presence of applessdstatistics and the fact it is running, but the fact it results in high CPU usage. applessdstatistics has been running in the background of Macs with SSDs installed for a long time without issue until macOS Sierra 10.12.2 was released. However, after this release it has resulted in high CPU usage and, in some cases, a noticeable amount of noise coming from the machine.


Notes: 


The 2nd issue (re high CPU usage and noise levels) is also affecting some non-SSD Macs affected by the first issue.
It is my understanding that Apple has an open ticket on this issue (i.e. I think it's the one ticket for both issues as they're related) and that a future update will resolve this.