The estimation of whether one is fat or thin just by looking is difficult, tall people can hide their weight better and short people look overweight. Hence the need to check one's weight accurately and regularly.
Keeping track of the weight is important not only to check for weight gain or obesity but also for the opposite. For instance loss of weight (not the intentional one byu dieting and exercise) would mean onset of diabetes,hyperactive thyriod,stress/anxeity or even tuberculosis and cancer.
Weight gain in children and pregnant mothers is normal or physiological, again this needs to be done regularly to check that the weight gain is healthy and not abnormal. More than a single reading alone, the trend of the weight (gain or loss) plotted against a time frame is more relevant.
One should not be obsessed with checking weight daily but we can easily have a bathroom weight scales and check our weight once a month or so. We should take into consideration of the fact that each weighing scale would be different.It's also useful bear in mind the fact that women tend to gain around 1.5 - 2 kgs just prior to their month cycles.