Early aircraft had mechanical brakes, and older aircraft with hydraulic brakes seldom have redundant, let alone brake failure warning light systems like a modern motor car.
During pre-flight walk-around, a cursory look at the brake cables, (if mechanical), checking for obvious leaks, the condition of the hoses/lines and the thickness of the pucks, if visible, was not just de riˇgueur but necessary.
I remember flying the Bristol 170 Freighter equipped with pneumatic brakes.
Regardless what the air pressure gauges read, it was considered prudent for the pilot to check the effectiveness of the brakes more than once prior to take-off. The moisture contained in the air of the aircraft compressor frequently froze before it could reach the brake shoes during winter at Arctic latitudes. Also remember only two wheels had brakes on the Bristol, not six wheels or more like on a modern airliner.
With all the redundant safety systems in a modern airliner, I personally consider testing the brakes during taxi a throw-back to simpler times, i.e. a tradition. One invariably has to brake the airliner prior to taking the active runway anyway..which I would consider a pre-flight test.
Here is a photo of an upgraded Beech 18 brake. To paraphrase Yogi Berra, "You can see a lot of things just by looking.."
