The suppliers provide (or are looking to provide) an electronics suite to car manufacturers. The car manufacturers want the system to be safe lest they be sued out of existence. One part of that will include contractual requirements for the system to have clocked n-kilometers on the highway in full (or partial) operation. For example, one project had a requirement for car(s) with full sensor data recording and partial automation enabled for 1 million kms.
The automotive suppliers will outfit a handful of, say 2019 model year test cars with the proposed sensors in the correct place and drive them around roads and highways in the specified conditions. Outfitting the cars can be expensive with prototype hardware, collecting the resulting data is a pain, and as a result the suppliers I'm familiar with run a (relatively) small number of cars for a lot of miles to record all that data.
The point of all this is to collect sensor data for resimulation as models are developed and trained. If an exceptional event occurs, they can modify the driving model, then "replay" the new model against all prior collected data to make sure the change doesn't do something unexpected elsewhere.
This process takes a lot of time (years) to pursue in this manner. What Tesla is doing is deploying the hardware in the field, then using the deployed systems to collect data to be used for the development of the automation platform. Instead of a couple of test mules they can use every single car they sell and let you drive it around for them while they record the results. Data collection that would take years can happen in weeks. This is a brilliant shortcut to the process and it puts them a couple years in front of the competition.