It is a methodology that has worked for centuries, by observers noting patterns of change against time, analysing the result and proposing improved design solutions.
If the season is wet, the garden is getting water from the sky, and the tank is not being emptied sufficiently, so in steady state it will be observed to be always full.
Ice cores have thousands of layers, each of which represents a winter season of snowfall, and trapped in these are bubbles of air, particle of space dust and pollen which reveal climatic data of the time.
Transient modelling is the basis of weather forecasting, of managing ecosystems, rail timetabling, managing the electricity grid, setting the national budget, floating currency, understanding traffic flows on a freeway, solar gains on glass fronted buildings, or even of checking the day-to-day transactions of one's monthly bank statement.
With the transient modelling approach, you understand the whole process better when the inputs and outputs are graphed against time.