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: