From both parties’ point of view the starting date should always be on or before the access date. As employer you are required to provide access to the site by the access date and if you do not do so, that will be a compensation event (see clauses 33.1 and 60.1(2)). The contractor is entitled to start work on site on the earliest access date (see clause 30.1).
The starting date is important because it is the date when the contractor assumes the risks stated in the contract (see clause 81.1) and insures against them (see clause 84.2). It is also the date which starts the payment processes (see clause 50.1).
If therefore the starting date is after the access date, there will be a period between the two dates during which the contractor is not at risk for damage it causes and does not have to have insurances against that damage. In addition, the contractor’s payments under clause 50 and 51 will be delayed by that period.