Exactly what Yawgmoth was is completely unknown. He far and away transcended mere deity status, would have been at least as powerful as the incarnation of a stable plane (had they been proven to exist), and was stronger than all known Planeswalkers. Some believe he managed to render himself the incarnation of his plane, boosted by the existence of a Planeswalker's spark at its core, though this still does not accoutn for his full powers. Others suggest that he had conquered multipel universes and overlaid them with Phyrexia as he did to Rath and Dominaria, except with utter finality, and usurped the powers of multiple incarnate planes, thoug this is still insufficient as no number of unstable or artificial planes should have been the equal of Dominaria or Urza. Exactly how he got so powerful was the major concern of the surviving Planeswalkers after his destruction, who believed that if this happened once it could happen again. It is the biggest remaining mystery of the Multiverse.
I have a simpler view. There was no grand plan or reason, he simply did the impossible. It was, in fact, his hallmark to do exactly that, as he proved again and again. Just as one in a million have the Planeswalker's spark, and one in a million of those will be ignited, one in a million have the dreamer's ability to simply not accept the impossible as impossible, one in a million of those have the vision to see that dream made reality, and one in a million, again, have the utter determination to see that vision through to the end. They are, I would like to believe, a naturally-occuring phenomenon of the Multiverse, a mathematically-certain Reset Switch, intended to kick in eventually before flaws in the Multiverse grow too numerous for it to maintain itself.