Adapter should be because different country uses different wall socket...Some people would buy the UK adapter because they prefer 3-pin over the 2-pin plug that is sold in Singapore...
People have the misconception that buying a Switch overseas equals to no warranty...It is not true at all...You can still claim warranty at the country you purchased the Switch from...If you go to Japan quite frequently, Nintendo Japan will not stop you from going to Nintendo Japan for a faulty Switch...Same applies to the Nintendo Switch that is bought in any other country...
Generally people with common sense would know there is warranty if they buy something (especially for something that cost more than SG$100) when they are in another country unless it is then thing is a "gray" set (exported by someone to another country). However, the service centre would likely be in the country the item is bought from.
Bring the topic back to buying a Nintendo switch in Japan, most people here would consider getting when they are there for holiday which means they do not go there often. Therefore if something were to happen to their device, it is likely not worth the time and effort of sending it back to Japan for servicing which also means no warranty. Too bad countries nearer Singapore usually do not sell such stuff cheaper than sellers in Singapore.