[Naruto]

I'm doing a reaction where the procedure calls to add one of the reagents slowly over the course of 30 minutes to the reaction flask, but why would you do this instead of adding it all at once?  If necessary, I can copy/paste the procedure here.  Thanks!

[Corribus]

It usually comes down to favoring a desired product or influencing reaction rate. E.g.:
(1) so that one reactant is always in large excess, which can favor, e.g., monosubstitution vs disubstitution
(2) if the reaction is very exothermic...

[Babcock_Hall]

@OP,
Corribus gave a good answer.  Another thing to consider: when a reaction is being held below room temperature, one wants to avoid adding a new reagent (presumably which itself is at room temperature) too quickly, or the reaction will no longer be at the temperature one desires.