I've spent the last several years working with this scale of robot at my start-up. The practical use is mobile phone, tablet, and general touchscreen testing. In some products, such as automotive head unit displays or any medical device with a touchscreen, there are legal and company policy rules requiring that functional testing of a device must match human actions as closely as possible. Which means you can't only test the device through USB or some other back-end developer interface. In other cases, performance/latency testing teams want to test a touchscreen device as a black box, meaning with no extra instrumentation running on the device-under-test, to get good real world performance data. In all these cases that means a capacitive tablet stylus, instead of a pen, is attached to the end of a robot like Cartesio.