Why do single electrical receptacles exist?
There are quite a few reasons to use a single receptacle instead of a duplex receptacle. A few that I know of:


Recessed for clock or behind a TV. In this case, it is sometimes easier to use with one centered receptacle instead of top or bottom of a duplex receptacle.
Air conditioning, range, dryer or other larger-than-usual circuit (in the US, this means anything > 120V or > 20A). I believe in at least some cases this is a code requirement.
Refrigerator or freezer in a GFCI-required area in order to avoid GFCI requirements (subject to local code and AHJ). In this case a second receptacle would be indication of intent to plug in additional devices rather than using a single receptacle for a specific exemption.
Cooktop ignition. I have this in my own house. My electrician installed a single receptacle as (a) there is no reason to ever plug in anything else inside the cabinet under the cooktop and (b) he installed it "upside down" - ground pin on top - instead of the more typical (for the US) orientation, for an extra little bit of safety (which would be awkward/confusing elsewhere but for the seldom unplugged cooktop is perfectly fine). The same logic would apply for a single receptacle for a garbage disposal.
Specialized sensitive equipment - e.g., medical or computer - to minimize interference from other equipment on the same circuit (e.g., there are devices that will reboot due to voltage drop if a laser printer starts printing on the same circuit, but which will be fine as long as they are on separate circuits - and any time you have an open receptacle it becomes "available" for a printer or vacuum cleaner or whatever).
Backup power - if you have a generator or battery backup with limited power then using single receptacles is a way to make sure that only the specified loads are on the automatically transferred circuits.


I am sure there are more reasons - if anyone has any good ones, feel free to add to this list (or make your own answer if you prefer).