I know that gravitational lensing is a geometric effect that occurs when light changes direction because a massive object causes space-time to warp and consequently the radiation rays change the path they are taking. And I also know that thanks to Gaia it is possible to predict gravitational microlensing events, but what would be the objective of predicting this type of event?
The objective is to calculate the mass of the lensing object.
Astrometric gravitational microlensing is an excellent tool to determine the mass of stellar objects. Using precise astrometric measurements of the lensed position of a background source in combination with accurate predictions of the positions of the lens and the unlensed source it is possible to determine the mass of the lens with an accuracy of a few percent. Source: Prediction of astrometric microlensing events from Gaia DR2 proper motions
By predicting future lensing events you can be prepared to make careful measurements during the period of gravitational lensing. This allows you to find the mass of the lensing star, which is one of the fundamental properties of a star, but one which can be hard to determine by other methods.