As mentioned, single-rank sticks put all of their memory chips on one side of the stick and leave the other side empty. Dual-rank sticks have memory chips on both sides.
Twice as much physical real estate allows for the installation of twice as much memory, but requires that the memory controller do extra work in order to determine which side of the stick the desired data is on (or needs to go on). The paths then need to get switched (much like rail tracks get switched) before the data transfer can take place. This takes time… and ultimately means that dual-rank memory needs looser timings in order to function reliably.
A very small amount of latency is sacrificed for a very large increase in capacity.
If you’re playing “Benchmarking 2019 (ePeen Edition)” then paying double is mandatory, and you will be mocked mercilessly by members of the Samsung B-die cult if you don’t. It’s not worth it for “Cities: Skylines” (or virtually anything else for that matter). Your performance bottleneck is nearly always somewhere else in your system.
The best exception I can think of: Ryzen APUs. Because Ryzen APUs use main memory, latency translates directly to FPS. With 2200G/2400G on a 3xx/4xx motherboard, the Infinity Fabric was also tied to memory frequency. Folks gaming on an APU would probably stand to benefit most from (low-latency, high-frequency) single-rank memory. In 3rd Gen Ryzen processors, however, IF and MemFreq can be decoupled — so high performance memory isn’t quite as important as it was before.