Jignesh Raiyani explains the purpose of the identity cache and shows how you might disable it if you want to:
In the release of SQL Server 2012, Microsoft has introduced a feature of Identity Cache. The identity jump doesn’t cause any issue neither with the database nor the running tasks; however, this identity value gap is not acceptable in some of the business-oriented scenarios. This Identity counter values of the columns are stored in the system table separately, which is known as sys.identity_columns DMV. This reference object sys.identity_columns does not rely on the transaction status, and it doesn’t matter that the transaction on the user table is committed or rolled back. Now, in case this user transaction is being rolled back, we will find that the identity value has been skipped in the sys.identity_columns table. The reason being is that the value of the Identity column value does not get rolled back. Apart from the above scenario, there could be various reasons or situations where these issues of the skip of the Identity value will be seen, which will be small in number and random. However, when there is an issue of Identity value jump, the amount of the skip value will be prominent and precise in number, depending upon the column data type, which is easy to identify.
My advice is, don’t disable this unless you have a really good reason to. “I don’t want to see numbers skipped” is a bad reason because identity columns never guarantee sequential insertion. You can still have skips due to transaction rollbacks or other errors on insertion, and the identity cache helps with performance problems in cases with lots of writers inserting rows into the table.