Garter stitch can also be incredibly boring and seemingly takes FOREVER to grow.
Seems like it would be the perfect test of perseverance for a potential new knitter. If I'm teaching someone to knit and they don't have endless persistence with it comes to knitting endless rows of garter stitch (and ripping out and starting again when they make mistakes), I typically think the learner won't ever become an earnest knitter. I know there are exceptions to this generalization, but it seems to be a good indicator.
On a more positive side, garter stitch is simple enough to help a new knitter understand how to create rows of loops that eventually form a fabric. Garter stitch doesn't roll or curl up. Garter stitch fabric makes it easy to count rows. Garter stitch has approximately the same row and stitch gauge, making it easy to pick up stitches and perform things like mitered or modular knitting. Garter stitch creates furrows that can easily be used for illusion or shadow knitting.
Finally, when my life is a bit full, garter stitch can be an incredibly soothing and rhythmic way for me to continue knitting.
A while ago, I was watching television and didn't have my knitting project(s) with me, so I picked up some handspun yarn and some needles and just started knitting a garter/biased scarf.
With all the medical tests and procedures lately, I've found this easy and rich-colored garter stitch project has been a joy to work on. And even with this much length on the scarf, it seems to be an endless task. I will, however, persevere.