Just like any other skin treatment it has to be adapted to your skin type. It is used to make your make up last longer and sometimes, treat skin imperfections especially oiliness.
All face, lids and eyelashes have their own type of primer. The "primer" which is, to me, the most well-known and applied is actually the one for lips: Lip Balm. Though nowadays, many lipsticks promise to leave your lips smooth and hydrated, it is very rare to find one as nourishing as a lip balm and with a staining power. I personally never did though I own some fairly hydrating lipsticks.
If you feel like wearing make up in a somewhat polished fashion but have sensitive skin or just want to protect it from make up directly applied on it; your best bet is on base primer which will work as a barrier in between your skin and other products.
I prefer salicylic acid bases to the silicone ones for my pores. The Silicone tends to clog the skin. It obviously hides wrinkles and wide pores but it also enables the skin to breath. If anyone of you ever saw Goldfinger, the James Bond movie, they will very easily understand how bad stopping the skin from breathing is. In it, a women is found dead all painted in gold which lead to "skin suffocation".Even if I mainly own silicone based primers, I don't wear much make up overall when I am not filming and often just make up my eyes so only eyeshadow primer is needed.
Once again though, if you do need to look impeccable at work, primers are your allie. They will prevent your make up from streaking, your skin from getting oily and will keep it from all the outside world's agressions (pollution).
My personal list