As documentation suggests here
If document have some fields which have less data compared to others like content field and if we want to retrieve only those fields, then we can use store. By setting store to true for those fields, we can get these data without extracting it from source. I tried to use this and I observe that size of index increases compared to index which has mapping without store attribute. So if it will increases the size then in what kind of scenarios, we can use it. Does it reduce the total response time of query? Can we index the content of file without saving it in document?