FabricA fabric prone to fraying, such as a loosely woven fabric, will require more sewing in the seam finish than a tightly woven fabric that does not fray.
Some fabrics do not require a seam finish. Polar Fleece is an example of a fabric that requires no seam finish at all. However in a garment, you may want to add a seam finish to tame the bulk of the seam.
Strength & Type of SeamIn most cases a crotch seam on a pair of pants will take more abuse than a side seam on the same pair of pants. It is advantageous to secure the crotch seam with a seam finish that will strengthen the seam as well as prevent fraying.
A second row of stitching as in a clean finish seam finish and a zigzag seam finish will strengthen the crotch seam and prevent embarrassing moments.
Visual AppealAlthough seam finishes are inside the garment there are times where the seam finish will be seen. Think of an unlined jacket that is worn open in the front. Many times the inside of that type jacket is seen.
A seam finish in an unlined jacket would need to appear finished and have visual appeal more than the seam in a lined jacket.