To answer the second question: yes, there is a very definite difference in texture when you crochet in the front loops only versus when you crochet in the front loops only. Crocheting in the front loops creates a line of stitches that are flat against the fabric, while crocheting in the back loops creates a ridged line of stitches that stand out from the fabric.
Now that this difference is pointed out, the first question can be answered quite simply: a designer may chose to crochet in the front or back loop only when they are looking to create a different texture for their finished project. Here are some general guidelines to help you decide which loops to use.
Front Loops Only: Crochet in the front loops when you want to add some stretch to your crochet fabric (such as when making garments) or to add a decorative touch to what would otherwise be a solid panel of standard stitches.
Back Loops Only: Crochet in back loops only for maximum texture change and elasticity. Crocheting in back loops only of solid single crochet is a popular method for mimicking knitted ribbing.