defragging decreases the life of a hard drive?? I did it for years.... or do you just mean SSD?
Just for SSD, the difference is that a regular hard drive doesn't really deteriorate from rewriting files, and since it physically has to locate files it helps to improve speed by defragging so that it can better organize the data so it doesn't take as long for it to locate the files. SSD doesn't have that issue since it can go directly to the file location, but it does wear out over time from writing files.