Can anyone tell us what Johannesburg(South Africa) was called previously and what was the reason Johannesburg grew from its roots?

The city is built on land that was originally a farm called Randjeslaagte.
It became a small town after the discovery of gold there in 1886. The city was named Johannesburg after two commissioners who confirmed the discovery of the gold.

http://ctct.essortment.com/historyofjohan_rtbh.htm

I would also have accepted Witwatersrand
which is the area name (still exists)
info here:
http://www.1upinfo.com/encyclopedia/W/Witwater.html