This type of lock is used when the application program intends to read data and potentially change the data that is read. This means the data may or may not be changed. In this case when you lock the a record with xclusive lock then no other application program is allowed to change it. So if you program decides that the record should not be changed then you restrict other applications from changing the data.
In this case you let the system know that you application programs has intentions to update a record base don certain conditions. Here you allow other applications to read/update the same record that your program is interested in. If your application wishes to update the record then the optimistic lock is converted to xclusive lock.
You should be able to find more info How Optimistic Locks Work - SAP Lock Concept - SAP Library.