What do they do for the sound of a guitar, and what traits would you want in one you are considering to buy?

There are tons of different types of pedals. What kind of pedal are you talking about?
*Taylor 214ce Acoustic
*Fender Highway One Honey Blonde Strat (Fralin Pickups)
*Epiphone Zephyr Regent (Gibson '57 Humbucker)
*Vox 847 Wah
*Fulltone Fulldrive 2 MOSFET
*Fender Blues Deluxe Amp
Exactly. I do not know what I would want one for, and what pros use them for. I mean if you are an accomplished musician what can you gain from a pedal. I have seen the ads for all the different types of pedals, ansd I just want to know which traits are valuable.
Personally, if you dont know anything about them, go to your local shop and try them out, that'll be better than us trying to explain it in words.
You buy a pedal because it changes your sound.... that's why. You would look for a pedal that can change your sound. Whateva is valuable depends on personal preference... try one at a shop.
try them out there are so many different pedals out there.
Quote by Dudage
Pussies play P-90's

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alright i'll try it
but if it falls out and my guitar breaks i'm going to eat your first born.
...just so you know
effects pedals help you creat sounds and textures that you can't get from just a guitar and an amp. I suggest you read this column

"There he goes. One of God's own prototypes. Some kind of high powered mutant never even considered for mass production. Too weird to live, and too rare to die."-Duke
Well there are delay pedals, distortion, chorus, flanger, phaser, wah wah's, etc. The list goes on. They all have different effects.

A delay pedal basically is an echo effect. Distortion is pretty self-explanatory, distorts the guitar for that rough sound. Chorus, flanger, and phaser effects are all similar and basically make a wavey type of sound. It's a little difficult to explain. I'd suggest going to your local guitar shop and trying some out.
*Taylor 214ce Acoustic
*Fender Highway One Honey Blonde Strat (Fralin Pickups)
*Epiphone Zephyr Regent (Gibson '57 Humbucker)
*Vox 847 Wah
*Fulltone Fulldrive 2 MOSFET
*Fender Blues Deluxe Amp
they change your sound, take your sound to new demensions pedals allow you to get more creative with your sound, make a more original sounding songs i could go on for hours
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I like this guy, he's UG's Greek, and he just told your ass in two paragraphs. And I once spent 5 minutes watching his avatar.

A Brain Malfunction

We'll Never Admit As Defeat
ther are overdrive/overdrive pedals, modulation pedals, delay pedals,

experiment with them, you dont need them, but they can add interesting textures to your songs. there are no pros and cons of using pedals, its all up to what you want. If you are all about the straight 3 piece punk, then you probably wouldnt use much effects at all, so its up to what you want. If you dotn know what you want, then you either dont want any, or you havnt been exposed to it before. Listen to some of your favorite records, almost every band uses some sort of effects.

Gibson Les Paul Studio w/dirtyfingers pickups
Gibson Joan Jett Melody Maker
VOX ac30 head
Marshall 1960 4x12 cab
Fender Hot Rod Deville
You dont necessarly "need" pedals, but they're fun to play with. If your looking to add something to your sound check out a few different ones. www.musiciansfriend.com has sound clips of almost every pedal they carry.
My Gear:
American Deluxe HSS Strat
Crate GT212
Vintage '66 Fender Champ Amp
Dunlop Crybaby 535Q
Jekyll & Hyde Ultimate Overdrive
Vintage '74 MXR Script Phase 90
Vintage '77 MXR Flanger
Orange Squeezer (clone)