Freezing a Gin glass means promoting ice stones to touch walls of the glass in a circular motion. Albeit unintentionally, some of the ice will pass into liquid making the glass accumulate some water. For Gin and Tonic, we just want to remove the water and not the ice. To facilitate this process we can use a strainer. Relying on the edge of the glass we can reverse it and outputting all the water on it at the same time that we retain the ice cubes.
The strainer may also be useful in the production of cocktails. Its main function is to always make the separation of the solid from the liquid elements, allowing the latter to be poured into the glass leaving the solid, unwanted on the drink, in the shaker or mixing glass.
For more creamy finishes, where we do not want small ice crystals, we can use a net strainer beyond the strainer. This is called double strain.