WiFi & Ethernet Both On, Any Benefit / Drawback?

It does not hurt if it is on.

The default System Preferences -> Network -> Interface (left column) -> Gear menu -> Set Service Order always has Ethernet above WiFi, so if you have Ethernet plugged in, then Ethernet will be the interface used for all your network traffic. If you unplug your Ethernet, then it will switch to WiFi.

Since you have an iMac, chances are you will not be picking up your Mac and going to another room with your iMac in your lap, or going out on the patio in a lunge chair 😀, so you could turn it off.

HOWEVER, if you are going to use AirDrop, I think that depends on creating an ad-hoc WiFi connection between your Apple Devices.

If you get an Apple Watch and want to allow your Apple Watch to unlock your iMac, you need WiFi turned on. It is not used for normal network traffic, but the Apple Watch seems to care.

I'd disable it.

Notes...

System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.

The interface that connects to the Internet should be dragged to the top of the list.

System Preferences>Network, unlock the lock if need be, highlight the Interface you use to connect to Internet, click on the advanced button, click on the DNS tab, click on the little plus icon, then add these numbers...

8.8.8.8

8.8.4.4

Apply.

With Respect to your WiFi traffic. You can use the built-in macOS WiFi Diagnostic to scope out all the WiFi base stations and which channels they are using, and how strong their signals are, then change your WiFi base station to use a set of channels that does not seem to be over used in your apartment. NOTE: as you do not control your neighbors, they could switch channels on your and one day you have good service, and the next day, you are using Wireless Diagnostics to find unbusy channels again 😀

Hold the Option key and Click on the WiFi menu bar icon

Select "Open Wireless Diagnostics"

Go to Wireless Diagnostics -> Window -> Scan

Expand the window so you can see all the columns

Click on "Scan Now" in the lower left corner

The bold entry will tell you what WiFi network you are connecting to.

If you have both 2.4GHz and 5GHz WiFi, then you will have your other channel listed as well.

All the others are your neighbors

Look at the RSSI (signal strength), the Noise, and the Channel columns. These are the most interesting to you.

RSSI with a low negative value -30, -40, -50 is a strong signal.

Noise with a high negative value -70, -80, -92 is "Great" as it says there is very little noise on that channel.

What you want is to find a channel where the neighbors report high negative number, meaning in your apartment their signal is weak and will not interfere with you.

For 2.4GHz channels 1, 6 and 11 are best, as each 2.4GHz channel actually uses up 5 channels of bandwidth, so channel 6 spans channels 4,5,6,7,8 Channel 1 spans -1,0,1,2,3 and Channel 11 spans 9,10,11,12,13. For 2.4GHz it is best if you can find a channel where the main number all the neighbors have high negative values, and the 2 channels above and below also have high negative values (or are not being used).

Then find a channel that sits in the middle of an area where the neighbors signals are weak and you would be strong.

For 5GHz each channel does not overlap with its adjacent channel, so just find the channel where the neighbors have high negative numbers.

Or just buy the biggest baddest, WiFi Router you can afford. The Netgear Nighthawk is reported to be that kind of modem 😀

At least you know what your situation is.

A few years ago, I spent a month in a Florida condo working remote (Wife wanted to get away that winter), and I found that the condo WiFi was 2.4GHz only. I also found via a Wireless Diagnostics scan that most of the other condos were 2.4GHz.

Since I really wanted the Apple Airport Extreme 802.11ac router (the last one Apple made; before they stopped making it), I visited the local Tampa Apple Store and got one, and use 5GHz because it was not a busy frequency in the condo building.

So I will at least ask if your WiFi router is both 2.4GHz and 5GHz capable? If not, you might want to consider getting a modern WiFi router, if only because if you have an iPhone, it will be able to use you home network more easily.

Yes, disabling it would be best so there's less interference for your other devices that might rely on wifi (iPad, iPhone, Smart TV, Gaming Console, ...) and perhaps for your neighbours (if you care).

FWIW, Location Services requires WiFi enabled to work correctly.