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