The upside circuit break has very practical implications. China's current stock market was started in 1990. The general public began to paying attention to stock market around 2000. From the historical point of view, both the stock market and the general public's understanding on it are still in early stage. So the attitude on stock market is very speculative. Coincidentally, the economy in China has been undergoing stunning growth during this period, which generates a huge amount of wealth. This furthers the stock market's speculative nature. As a result, the volatility could be very high. It was not uncommon to see some stocks undergo several +/-20% changes during a week, before the upside circuit breaker was established. Its primary purpose is to curb the speculation.
Limits are subtly different from circuit breakers in that trading can continue... within the limits.
[1] http://www.cftc.gov/industryoversight/marketsurveillance/spe...
https://en.wikipedia.org/wiki/Trading_curb#China
"If the CSI 300 Index rises or falls by 5%..."
Also, when people have short positions in the equity markets they get margin calls and are forced to close out their positions by buying stock when the market rises. This would be a similar process that roymurdock mentioned but to the upside. It's called a "short squeeze."
Conversely, there is no mechanism that can push the market increasingly higher, other than hyperinflation or a collapse in, say, the bond market which might theoretically drive investors into equities. But if the bond market collapses, the world has much bigger fish to fry and the stock market will probably come down right afterwards.