Without it, endless questions would have no answer from God, or would need infinite idle decrees
Besides, unless we ascribe such knowledge to God, countless questions could be put to him, like the ones David asked, to which God could give no certain and clear answer except that the matter cannot be known. That seems to take a good deal away from God's omniscience.
Suppose someone asks what a man who died peacefully in Christ would have done if martyrdom had been offered him. Or whether this or that man, had he sought the hand of a certain girl before the man she married, would have won her consent or not. Or how a certain person would conduct himself if he were elected king of Poland. There are endless questions of this kind.
Shrewd and experienced people can make guesses about such things, so it is unlikely that God is simply ignorant of them. It seems unworthy of God not to know for certain what a man can reach by probable conjecture.
Someone may say that God does know such things and can answer distinctly and clearly about them, but that such knowledge presupposes a free decree about whether those things would come about in this or that case. Then it will follow that there are in God countless decrees about things that will never actually happen. Those decrees would be pointless, empty and idle, which is quite absurd.
Latin
XVIII. Adde quod, nisi talem scientiam Deo tribuamus, infinitae poterunt quaestiones Deo proponi, ad modum illarum quas proposuit David, ad quas Deus nihil certum & perspicuum respondere poterit, nisi rem esse quae sciri non potest; quod non parum divinae omni scientiae detrahere videtur. Exempli gratia, quaerat aliquis, quid facturus fuisset iste, qui placida morte in Christo decessit, si oblatum ipsi fuisset martyrium? An si iste, vel ille talis puellae quae alteri nupsit, prior nuptias ambivisset, assensum fuisset impetraturus nec ne? si certus aliquis in regem Poloniae eligeretur, quomodo in illo regno sese gesturus esset? & infinita hujusmodi, de quibus cum homines sagaces, & rerum periti conjicere possint, non est verisimile ea simpliciter a Deo nesciri. Ac indignum Deo videtur, illum non certo compertum habere, quod homo probabili conjectura assequi potest. Quod si quis dicat, Deum talia quidem scire, & de iis distincte & perspicue respondere posse, sed talem scientiam praesupponere liberum decretum de rerum istarum futuritione, in hoc vel illo casu, inde sequetur in Deo esse infinita decreta de rebus nunquam revera futuris, ac proinde frustranea, inania, & otiosa quod valde absurdum est.