Using of "Commit" method will expilictly save the data into the database.
As a best practice Commit method can be used when we use multiple Obj-Save methods in an activity.
For Ex: I have five Obj-save methods in an activity, I dnt want to save data with each Obj-Save method, but I want to save the data after execution of all 5 Obj-save methods. For this scenario, I will simply have 5 Obj-Save methods without checking "WriteNow" and I will use "Commit" method after the all the Obj-save methods within that Activity.
We can also avoid using "Commit" method if we dont want the data to be saved into DB immmediately.
Please find the screenshot attached, may that gives you clear understanding.