The FILLFACTOR feature optimizes the utilization of space for data pages by
reserving space for the expansion of existing records. When a record grows up and the page free space is not enough,some records will be put into other page, which affects the performance seriously.
So we set FILLFACTOR to avoid this.The principle is when the length of the records changes, the length of all the records won’t over the size of the page, avoiding some records will be put into other data page.