A fire sale can be seen as a strategic move. The goal of a fire sale in fantasy football is to give away your best players to a team that is in a more competitive spot than your own. If your team is near the bottom of the league, you would make trades with teams that are near the top. This is supposed to help out both teams.
A move like a fire sale makes the most sense when you are in a keeper league of sorts, where you get to keep a certain amount of players at the end of the season for the next one. The reason it makes more sense is because you could trade players who are playing very well to teams that can win the championship. In return you could get players that are younger and, while they may not be having amazing seasons, can help you in the future.
In regular season-to-season leagues, where it's a fresh start every year, fire sales take place simply to help out teams that can win in the playoffs of your fantasy football league. If you have good players and are not going to compete for the championship in a given season, take advantage of other teams' desperation in order to get good value for a trade.