Distortion- Pedals vs. Amps

I've only been playing guitar for a couple of years, but I still can't figure out what the point of distortion pedals are. It seems like the options for amps' gains are so overwhelming, why would you buy a distortion based pedal? I'm playing through a Marshall AVT-150 combo and it has four channels. The OD-2 being the dirtiest, down to an acoustic simulation channel. I borrowed a pedal from a friend once, and couldn't understand the reasoning, especially running the pedal through the OD-2 channel. It just put distortion x distortion = lousy noise. Could someone enlighten me on the subject? What are the pros and cons of distortion based pedals?

You will appreciate this famous rig then: http://guitargeek.com/rigview/312/

You may not realize that you're already (more or less) using pedals.

The Marshall AVT series has a single tube for distortion voicing, JUST LIKE A
PEDAL. It's not even running at true tube voltage, JUST LIKE A PEDAL. You have different channels with different gains and EQ's, JUST LIKE PEDALS. The acoustic simulation channel is JUST LIKE A PEDAL.

You're right about adding distortion on top of distortion. It usually sounds very bad. I don't use distortion pedals. I use real tube amps and/or preamp/power amp rack rigs that have their own distortion sounds.

The best use for a pedal is to rely on the pedal's sound and run the amp clean.
I just don't care for the sound and noise of pedals, I prefer tube amps. Guitar

Typically when you use a distortion pedal you run it through the clean channel. They can be useful for amps with only one channel and with tube amps that have no master volume control

You bet! I happen to own the 2204, the 50W version of the same Marshall. But I prefer my old Hiwatt 4x12 Fanes cab to the Marshall cabs.

I also have a second Hiwatt 4x12 that is newer, brighter and more efficient. I use it with various tube preamps into a Peavey Classic 60/60 tube power amp (60W RMS x two, all tubes, black plate RCA 6L6GC). ...or my Matchless Spitfire circuit built out of an old hi-fi amp. ...or an old Stromberg Carlson PA amp with octal preamp tubes.
...or my new find--stereo single ended tube amp out of an old reel to reel. ...or my
Hughes and Kettner Cream Machine.

You get the idea. All REAL tube distortion. No pedals or emulators for me.

Just say NO to sand distortion!