initial rate

(1) The instantaneous rate is defined as:
d[reactant]/dt
where [] is the indicator of concentration.
Therefore, to obtain the initial rate (which is also instantaneous), time t has to be small.
(2) Since for all reactions, initial rate is the fastest along the reaction coordinate, i.e. the rate of reaction decreases with time owing to continous decrease in concentration of reaction, for fair comparison and best significance, initial rates are preffered to those at other times.