What effect did Magmortar have on the metagame?
It's more powerful than Charizard, has the best coverage move a Fire-type could really ask for (Thunderbolt), and could run 4 different items effectively (Specs, Scarf, Expert Belt, and Life Orb). Magmortar was probably the hardest Pokemon to switch in on in NU history without already knowing what item + Hidden Power it was running. Once you knew the set, it was fairly easy to revenge or deal with, but before then it was extremely good. Magmortar did well vs bulkier teams that relied on switching, especially because aside from stage 0 there weren't any good Spikes setters that could hinder Magmortar switching in and out (though it is SR weak).
In what main roles was Magmortar used?
Magmortar was one of the premier special attackers and the best Fire-type during its time in NU. It had enough speed to run a competent scarf set (outsped by Sawk and Rotom-S but faster than Braviary and co), was powerful enough to wreck defensive cores with Life Orb or Specs, and could run a good lure set with Expert Belt and the Hidden Power of your choice (usually Grass for Quagsire, sometimes Water in Stage 0 for Camerupt/other Magmortar). Heck, Mach Punch could even 2hko Absol with no investment and a neutral nature, while avoiding Sucker Punch. If I had to compare Magmortar to a Pokemon that's in today's NU, I guess I would call it Charizard on steroids that didn't take 50% damage from SR. Base 80 was the speed tier to hit in the earlier stages of NU, and Magmortar outsped them all, like Charizard does to base 95.
What caused Magmortar to have a significant impact?
I've said it before in places, but let's recap: Highest Special Attack in the tier (Glaceon was PU), high base power STAB, excellent coverage (Fire Blast, Thunderbolt, Focus Blast, and HP Grass was essentially perfect), handy but not overpowering speed (Base 83 was more than usable at the time), usable special bulk for a sweeper (75 hp / 95 spdef is pretty good for NU standards). In Stage 0, Magmortar's "counters" were Slowking, Lanturn, and Quagsire mainly. After Stage 0, that list dropped down to: Quagsire, Thick Fat Miltank, and Altaria (I suppose you could call Regirock a check but Focus Blast took huge chunks out of it). Decent, but definitely able to be taken advantage of by teammates.
How do/did you deal with Magmortar in NU?
Pray you didn't get hit by a crit Thunderbolt/HP Grass when bringing in your Water-type? I mean like I said there were checks, but Altaria is really the only mon that could switch into every move standard Magmortar had. Really, the easiest way to deal with it was by revenge killing. It was pretty frail on the physical side, and if you could get down SR you could keep both Magmortar and Cryogonal/Armaldo (the main spinners) at bay. In addition, Absol was around at the time, and Sucker Punch hurt a lot. In the later stages that Magmortar was around, Samurott started coming into play, and Aqua Jet did a ton of damage to it. The metagame was extremely offensive, as you can imagine.