I just came across an amigurumi pattern that specifically asks for a single crochet into back loop only and it made me question what the difference is.
So, what is the difference between normal single crochet and single crochet into back loop only?
Most amigurumi patterns are worked in the round, not flat. For flat items the difference between regular and back loop only (blo) crochet stitches is striking, it creates horizontal ridges, which are often used to imitate ribbing, like in this sock:
In items worked in the round the difference is more subtle: a horizontal line appears between rows of stitches, as in this ilustration:
For the most part this is a decorative element, so you can include it or not, as you feel like. Try both kinds of fabric and decide which one you like more.
Sometimes this is a functional element. Toys are usually evenly stuffed, so the feet are rounded and working one round of blo sc will give a gentle emphasis showing where the sole ends and the top of the foot begins, like here, in my own work from last year:
The end result looks quite different. If you work an entire row that way, it makes a ridge, whereas normal single crochets (using both loops) work up flat.
Here's an image from a nice crocheting guide that shows the difference between the two when worked in a swatch:
The guide notes:
Structurally, these fabrics are really different. Standard single crochet has a sturdier, denser feel to it. It has a smoother, less textured appearance.
In contrast, back loop single crochet has a stretchier, more fluid feel to it. The appearance is much more textured, as it has interesting ridges on the surface of the work. If you rotate this fabric on its side, you can take full advantage of the stretch and mimic the look of a knitted rib stitch.
While amigurumi often use single crochets, when you want a ribbed texture, like with this cactus project (requires Ravelry account), you need something different.
(Pattern here)
This stitch is an easy way to achieve it.