The thing about soap bars is that it can be a real pain to finish up completely, which is why most folks tend to chuck away instead.
Instead of wasting all that precious soap, Redditor localguy69 recommends turning all those leftover soap bits into a shower gel. Just boil some water, dump the soap bits in a food processor, add the boiling water, and blend. Once you're done, you'll have your homemade shower gel which you can then stick in a bottle and use to get yourself clean. And don't forget to rinse out the food processor, unless if you like suds in your food.