Call them what you will: resistance rolls, saving throws, defenses, or what-have-you; they’re all the same. This is the mechanic that games use to prevent the impartial nature of probability mechanics from killing characters through sheer dumb luck. After all, a roleplaying game is not reality. Happenstance is a great tool to keep the rules fair, but not when it would chill the fun. This is what these rules are designed to curb.
Let’s face it, random character death is annoying as can be. Players are likely to be much more upset if there is no meaning to a fatality in the ranks. The attachment is greater the more experienced the character is and the longer it is played. This is why games have a built-in system to stave off oblivion. It isn’t just a technique for writers, after all. Saving throws are an RPG’s safety net to protect it from its own math.
One of the salient features of roleplaying games is the storytelling element. The gamemaster describes a situation and the players respond. This call-and-response mechanism is the core of the interactive nature of the tale woven by a group of players. The math behind the dice rolls is meant to provide an impartial judge so that arguments about who can do what are kept to a minimum. But it is this same randomness and impartiality that can lead to character death in an otherwise nonlethal situation.
So, what the resistance roll allows is a way to temper the probability which could kill characters undramatically. But that isn’t the sole reason for including them in a game. They also fill a dramatic roll in a game’s rules. Granted it is still a way to skew the math in the favor of the player characters, but there are times when players place their characters in harm’s way. Often these instances are climaxes in adventures. The heightened drama is rooted in how close to death heroes can get without crossing that threshold.
Regardless of whether story or probability motivates their inclusion, saving throws keep characters alive. And in systems where the chances of success improve as the character gains ever higher levels of experience, the level of danger such individuals can face becomes more intense. There is also the added benefit that the character won’t fall victim to a bad roll or two. If you consider how much time and energy gets invested in high level characters, it makes more sense why the math works this way without recourse to the dramatic story elements of this form of gaming.
What’s to be taken from the above? Chiefly that the safety net against probability’s cruel impartiality is also designed as a dramatic tool. As such, systems can be used to mitigate probability from hijacking the story and hence lessening your fun.