What is the purpose of tetrazole in oligonucleotide synthesis?

The [ATDBio book on nucleic acids](https://www.atdbio.com/content/17/Solid-phase-oligonucleotide-synthesis) describes tetrazole as an activator required to couple nucleotides. 



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> 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.
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[![enter image description here](https://i.stack.imgur.com/sLoKZ.png)](https://i.stack.imgur.com/sLoKZ.png)


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)?


