The ATDBio book on nucleic acids describes tetrazole as an activator required to couple nucleotides.
The diisopropylamino group of the nucleoside phosphoramidite is protonated by the activator, and is thereby converted to a good leaving group. It is rapidly displaced by attack of the 5′-hydroxyl group of the support-bound nucleoside on its neighbouring phosphorus atom.
The first molecule in the scheme they provide is clearly already protonated; a second step in which the diisopropyl amino group is replaced by a tetrazole molecule is displayed.
What is the purpose of this step; why doesn't the support bound nucleotide attack the phosphorous, directly? Are the arguments purely kinetic (i.e. sterics and availability)?