The stacks logs are files from your iPhone that are sent to Apple. These files contain data logs from apps, Siri and Dictation and Safari. The main reason of the logs is for Apple to improve their product performance including Siri and Dictation by reading the information from the logs.
The sysdiagnose file includes reports about the state of your device. The reports depend on what iOS firmware you are on and what device you are using.
The stacks logs and sysdiagnose files are not needed to be shared with Apple as they can increase the draining of the device battery. To stop the file transmissions which can effect battery life, go to Settings > General > Privacy > Analytics & Improvements then toggle off the Share iPhone Analytics and the Improve Siri & Dictation.