Asking for a modification of your divorce decree isn't necessarily unusual, depending on the reasoning. Many parents ask for modifications to adjust child custody, for example. Others may ask for modifications of alimony when they're getting remarried or have a change in their circumstances.
When you seek a post-judgment modification, you're asking the court to modify the final judgment previously made. For instance, if you have a child custody arrangement in place that doesn't work any longer, you may ask the court to modify that arrangement. If you have a property division settlement agreement that you want to have altered, you can ask for that as well.
It is a huge relief to finally get a divorce settled with the appropriate parenting and spousal support agreements in place. A couple of years down the road, however, the settlement that you negotiated may need modification.