I use a variety of stitches for several reasons. First, depending on the way the material is fractured a particular stitch will work best.
Second, some stitches are more prominent than others. Sometimes I want a stitch that is more substantial like the buttonhole stitch and sometimes I want a subtle stitch like the seed stitch.
Third, a variety of stitches just creates more interest and surprises. I want people to really examine my work and not just glance at it and move on to the next artist's work. That is particularly important if you have more than one piece of work and want each of them to be examined.
Fourth, Val who is a very wise artist told me that you want your work to be interesting from three distances. First from a distance so people will see it from across the room and cross to see it closer.
Generally people will only cross the room and come within a few feet or yards from the work. If it is no more interesting from this mid-distance then the person will move on to the next work of art. That means the art work has to have new things to see from a mid-distance or from a few feet or yards.
If the mid-range distance has new things to see then the viewer will want to get closer to see if there is even more to see. Now is the time that the subtle color, textures, and designs are seen.
That is a successful piece of art.