The temperature must be controlled, because if the high temperature caused coke and catalytic cause loss of unwanted products. Or potentially dangerous and damaged reactors.
If you need to control the temperature, high temperature will make the coke and the catalyst loses its ability to produce unwanted or potentially dangerous and cause damage to the reactor.
To control the temperature of high temperature will occur if the coke and catalyst loss can be byproducts.Or potential dangers and make the reactor damaged.