So you destroyed that Commander or lost a battle on Musket Bridge. The match is over so why would you keep the game list item around?
There are two good reasons.
1) The Re-Match button only exists on finished matches.
2) You can learn from the battle via the ‘full replay’.
Now most of the time these finished matches are trash-worthy, but there are a few that have distinct value. For example for ELO points matches must be RANDOM. Say you want another fight against ‘tin bucket’ and don’t want to wait for ages for another lucky random matchup.
By using the ‘Re-match’ button you can continue a series ELO scoring ‘random games’ with 1 person. After you make the new match though, you can then delete the completed match unless there was some ‘full replay’ content that you wanted to re-visit before deleting.