Rematches have always been an important part of pro wrestling. When a match draws major money, the first thing on any promoter's mind is how the wrestlers involved can face off again and hopefully draw even more. It is a business, after all.
Unfortunately, sometimes a rematch just doesn't live up to the first bout. A few major examples jump to mind - such as the second encounter between John Cena and The Rock, for instance. While a less-than-stellar rematch doesn't really taint the power of the original bout, it does stand as a disappointing corollary.
By contrast, a great rematch of a good contest makes for a stellar feud, and when an improved match follows a disappointing bout, the wrestlers involved get a chance to show how much they've grown or what they can do under different circumstances.
Some contests don't get rematches - which is a shame. There are plenty of times where two wrestlers could easily outdo their previous match (or matches) with one another if they were only given the chance. Again, they've improved over time, or storylines have changed, or fans take the talent more seriously after a while.
If these matches happened again in today's circumstances, they'd probably eclipse their forebears.