The whole smart home concept is about you controlling as many things in your house from your smartphone over the internet. A smart switch / plug-in smart outlet is one more addition to the smart home concept.
Good smart-switch outlet adapters allow you to add smart control (meaning: you can control them via wireless from your smartphone) to your existing outlets.
Good smart outlet adaptoers wok with multuple smart-home protocols (for example, Amazon Alexa, Google Home, and IFTTT) and devices. They also come with useful apps.
The best plug-In smart outlet: Belkin WeMo Mini (compact, for both Android and ioS, supports Amazon Alexa, Google Home, and IFTTT)
The second best plug-In smart outlet: iHome iSP8 (support for Apple HomeKit, SmartThings, Wink, and more)
The best affordable plug-In smart outlet: Geeni Energi (app for Android and iOS devices and Amazon Alexa support)