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Index fragmentation happens when a new record is inserted into a current database.
It is like adding a new name to a phone book, there is no room and the name has to be placed out of order.
Or taking a name out of a phone books, then there is a blank space, making the phone book longer than it needs to be.
Therefore taking longer to read it, back it up, and recover it.
If you do not defragment your indexes, your database has to cache all of those blank records, wasting memory.
Many defragmentation scripts are set to run as part of a maintenance routine, whether they need it or not.
This causes extra items on the transaction log, the longer the backups take, the longer the restores take.
Spotlight on SQL Server alerts you when defragmentation needs to occur
Bad internal fragmentation (a lot of empty spaces) and bad external fragmentation (a lot of records out of order) make the index longer than it needs to be and therefore storage performance is slower.
Setting a lower page fill filter (such as 50%) makes your inserts incredibly fast, but your reads are twice as slow.
Tweaking this fill filter is a dangerous game
Spotlight on SQL Server will show you if the root of your problem is the fragmented indexes and how critical they are requiring an entire rebuild or just a defragmentation.