You can use these to capture cyclic trends . So for eg. icecream sales . icecream sales will always have cycles or seasonality i.e higher in summers and lower in winters .
If i want to know the trend of icecream sales i.e sales going up or down ( not cycles but overall) i would use regression . it will give me a straight line slanting up or down
but i would like to know the cyclic trend i.e going up or down with the summer cycles and winter cycles i could use trigonometric ratios . it will give you a spiral trend line going up or spiral trend line going down