Why?
1) it helps to protect ur skin from the makeup to clot ur pores.
2) It has UV protection
3) It can also be colored to counter skin tones that are uneven.
I have tried 4 kinds of makeup base, the best have to go to (in sequence):
1) Makeup Forever (clear color) Price: around $60

Although this product do not have the UV protection but it has special ingredients for anti aging! (OMG!) u're gonna look younger while protecting ur pores at the same time! and it is near perfect to give ur skin the glow and make it suitable for applying make up! * it is also for High denfinition camera!* perfectto for ur wedding day:D
2) Clinique (skin color) Price: Around $40

Actually rather than a makeup base its more like a sunscreen, u can feel and smell it off the tube. Anyway buy the one in skin tone, it is high coverage and leaves ur skin like u have a light foundation on. Great for lazy days where u just wanna go out w no make up. High UV protection, worth investing!
3) Natural Republic Price: $29.90

Currently trying and using this. It is fairly good texture is pretty much like makeup forever at a more affordable price :) And i got it at half price as they were having sales, their BB cream have the same effect and they're hotsellers, almost all gone from the rack! Anw this brand is from Korea and is represented by the Famous Rain! UV protection- PA25++
4) Maybelline Price: Around $7-8
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Don't think u can find this around anymore, anyway it doesnt do what its support to except to even out the skin tone. I used mine in green, cuz i have red sensitive skin. But this does not help with protecting ur pores much, and i think i might be a bit allergic to it cuz my skin condition was bad after usage. so i wouldn't recommend this product.
So girls, treat ur skin better and use some "protection" everytime u apply makeup :D