They were both bred specifically for GBU random double and triple battles for the same team.
Jigglypuff (not Wigglytuff) with it's Friend Guard ability gives it's teammates a 25% boost to both defensive stats. In doubles/triples, Light Screen/Reflect boost both defensive stats by 33%, so just having Jigglypuff on field is very close to having perma-dual screens. It's taunt fodder, but in triples, you don't run into taunters very often. All Jigglypuff does otherwise is play total defense for itself, and it's teammates until near the end of battle when it uses Perish Song.
Swampert relies on it's natutal bulk and moves that can hit multiple of the opponents Pokes at the same time (Muddy Water/Surf, and Blizzard), while it's egg move, Wide Guard, helps defend it's team versus attacks that target more than one Poke at a time on your side of the field.
If you want to actually see them both in action, on the same team, watch this 12 turn battle......
Battle Video# 36-19507-48162