K.....an amp MODELER (I'll say POD from now on, for clarity) is a digital approximation of the tonal qualities of a certain brand and make of head. It is NOT an effercts pedal, but can be used as if it were if you so desire. You can plug a POD directly into a power amp and play....you would not want to plug a distorion pedal directly into a power amp and kick out a few riffs. You COULD...maybe....but it's a bad idea. That's a key differece. You can think of a modeler as a VERY VERY flexible and tweakable guitar preamp. But, if you are in love with Marshall, for example, and have little use for anything but the Marshall sound, you will not want a POD. You cannot get a true Marshall sound from a digital approximation. What you CAN do is create your own sound exactly tuned to your preferences. Say you love Marshall, but could really do with less of that "boxy" (in my opinon...don't get all upset now)cardboard like sound. You'd use a POD to model a JCM 800 and tweak out all the crap you don't want to hear. Following that logic, the possibilities are nearly endless if you can invest time and attention to detail with a POD.
The down side is, many people will tell you that PODs sound like ass. Those same people will also say that nothin will ever be as good as, let alone better than, mic'ing a real guitar rig and that a POD is only good for sketching out ideas - if you want REAL tone, throw the POD in the trash.
Now, there have been several threads PROVING that many people who say the only way to get good tone is to mic a real amp, just cannot tell the difference between the two if the POD is set up correctly.
If you go about using a POD with the understanding that it is not supposed to be an exact copy of a JCM 800, but instead a highly flexible and customizable tool to CREATE your own sound, you'll be very pleased with what you can do with it.