As @MassGuy said, its used to find the range to the target.What is a rangefinder and what would TC be doing with this when observed by snipers?
An experienced marksman then applies the range to compensate for the bullet drop, possible wind effect, and in super long range / ultra sophisticated shooting, the coreolis effect from the earth's spin (spinning earth causes target to move while bullet travels).
But.... in this case, I think the range finder was far more amateur "looks cool, and the movie guys always have one" than needed. Horribly, the would be assassin was way too close to the target to need to worry much about bullet drop etc.
Though it can be said that it was a "good" thing he thought he needed one as it gave the security more time to notice something was strange.
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