Webserial let's Home Assistant users flash their ESPHome devices without downloading or compiling any software. WebUSB let Google update my Stadia Controller to a normal controller after they shut down their cloud services. It also offers firmware updates for some Pixel phones.
These are all quite useful tools. I've never used WebMIDI but it's older than the other Web* APIs. When you have a use case for them, the APIs are a lot better than figuring out a cross platform serial port protocol (or, more realistically, writing a Windows application and letting the Linux/macOS/Android users figure it out themselves).
WebSerial/USB/Bluetooth doesn't do anything unless you permit it to. If websites used this feature, you've clicked "okay" when mapquest.com asked to use your serial port.
These are all quite useful tools. I've never used WebMIDI but it's older than the other Web* APIs. When you have a use case for them, the APIs are a lot better than figuring out a cross platform serial port protocol (or, more realistically, writing a Windows application and letting the Linux/macOS/Android users figure it out themselves).
WebSerial/USB/Bluetooth doesn't do anything unless you permit it to. If websites used this feature, you've clicked "okay" when mapquest.com asked to use your serial port.