There are load testing task, when You need to emulate user behaviour on web. Usually this is done via implementing/running http request/response scripts grabbed from http record/playback approach,
for example using well known JMeter tool.
New modern frontend dev framework line Vaadin or GWT generate rich traffic between back and frontend so it becomes some enomerous work to create and support such scripts and most cases even not possible cause of communicative info is coded and not readable for parametrization. So the only way is to start-up and use real browsers.
Here Bali comes with smart solution, cause it can run scenario in multiple threads on dedicated browsers resource pool.