While conducting any experiment, one has to (try to) minimize errors or inaccuracies. One way to do that is to minimize the variables. While comparing environmental factors (or studying the effect of environmental factors), say on any reaction or object or organism, it is ideal if we take the measurements at the same day at the same time. This will minimize the variability in experimental conditions and thus the error.
For example, suppose someone is studying the effect of environmental factors on the rate of reactions (any reaction of your choice). The person will have the reactions going on at the same time and will take measurements simultaneously. Then the results can be compared more accurately and can be stated in terms of the various factors. Simply, the environmental factors, such as temperature, pressure, humidity, etc. are not constant throughout the day and hence while comparing the effect of these on anything, we need to ensure that the same experimental conditions have been provided.
Hope this helps.