How do you maintain a fill factor in existing indexes? Usually the data keeps getting added, deleted, or updated in the table due to which the fill factor is implemented during the index creation itself. Fill factor is not maintained after an index is created.
The changes in the future can be determined only when a new index is created on a table that has an existing data. It is beneficial to set the fill factor then itself.
Maintaining extra space on the data pages obviates the purpose of using the fill factor. The SQL Server would then have to split pages to maintain an amount of free space per the fill factor, on each page.
Thus, when data is added filling up the empty space, an index can be created and the fill factor can be re-specified distribute the data again.