First of all let me start by saying there are 2 popular forms of setting powders. Loose and pressed.
You can buy translucent powder or powder with color depending on what you are looking for.
They are generally used to set makeup, help with prolonged wear, decrease oil and shine, provide a matte finish and in some cases provide a smooth finish to skin which helps to slightly conceal fine lines.
I like to apply it right after foundation but there are no rules with makeup, I also know people that apply it at the very end of their makeup application.
I tend to use the loose setting powder at home with a big fluffy brush and the pressed setting powder with a sponge for on the go and travel as its less messy.
Setting powders aren't a must for everyday wear but for some girls like me its a step I add in daily to help set my foundation and keep me shine free..... that being said I really have to get better at remembering to take my pressed setting powder with me when I change purses especially during the summer months!
I hope this has helped answer some of your questions on setting powders ;)
What do you use your setting powder for most?
Which do you prefer? loose or pressed?