Back again! OP, the reason for the 30 day clause is just to smooth through the administration of your estate, but also to avoid having two lots of executorship expenses (and possibly Inheritance Tax).
If the clause isn't there, and Mr & Mrs X are killed in a car accident, but no one knows who died first, or if one survives for a week or two then dies, then Mr's estate passes, albeit briefly, to Mrs. Tax may be payable in two tranches, depending on their wealth, and there are two estates so two lots of executorship expenses, and there may well be different executors appointed for each of them under their wills. There's a lot of unnecessary administration and possibly also expense.
With the 30 day clause, it doesn't matter who dies first, if they both go within those 30 days. There is only one estate to administer and only one calculation and payment of tax. (The tax issue isn't so important now that theres is rollover relief for spouses)
When I explain this to my own clients, I just explain that the 30 day clause is there to smooth through the administration of their estates if one of their beneficiaries dies within a month after them, as the beneficiary is treated as having died before them.