The advent of the month of Rajab is a signal for us to begin preparing for the month of Ramadhān. If we start gearing up from the month of Rajab then upon the commencement of the month of Ramadhān we will have a set routine of 'ibādah that we can gradually increase as the month progresses. In this way we will be able to fully benefit from the blessed month.
If we intend to increase our 'ibādah only after the month of Ramadhān has begun then by the time we get into gear, many days of Ramadhān would have passed, and we would have wasted valuable opportunities.