Use this space for any keywords you did not put into your front end copy. These can be alternative spellings or synonyms or any other keywords that you don’t want mixed in with your sales copy for whatever reason. For example, if you use the word “headphones” on the front end, you may want to put variations like “head phones” on the backend. Keep in mind that you do not need to repeat anything on the front end.
I’ve also noted that if you keep the keywords in a logical order, they index for phrases better. For example “headphones for kids women men teens” will index for “headphones for kids,” “headphones for women,” etc.