Depends on the foundation, weather and the condition of your skin.
Some primers smooth out the skin, so are useful if you have large pores and deep lines.
Some also help with excess sebum, so are good for combination or oily skin.
Some will help the foundation last all day or through out high humidity and heat. So foundations that don't last long or hold up well under certain weather conditions would benefit from them.
I mostly wear BB creams, and thus far most hold up great without a primer. But I do apply primer to my t-zone due to combination skin and some large pores, so that it looks smooth and firm.
Some primers smooth out the skin, so are useful if you have large pores and deep lines.
Some also help with excess sebum, so are good for combination or oily skin.
Some will help the foundation last all day or through out high humidity and heat. So foundations that don't last long or hold up well under certain weather conditions would benefit from them.
I mostly wear BB creams, and thus far most hold up great without a primer. But I do apply primer to my t-zone due to combination skin and some large pores, so that it looks smooth and firm.