From Google Support:
Why to use page-level permissions
Page-level permissions allow you to..
Is the sentence "Why to use page-level permissions" grammatical?
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It’s a headline, first of all, where some grammatical rules are different anyway. So this is not a sentence, but a noun phrase:
(This section tells you) why to use page-level permissions.
That is, it tells you why you should use them. “Why to…” and “why not to…” are very common in headings to encourage or discourage the reader, respectively. The heading could just as well be:
Reasons to use page-level permissions
Most answers focus on the fact that the cited text is a headline / noun phrase rather than a complete sentence, but I think the real issue here is Why don't we normally include the infinitive marker "to" between initial "Why" and an infinitive verb form?
Since "sentence" is not a structural unit but is merely determined by punctuation such as a period, it's not really helpful to discuss whether it's a "complete sentence" in order to explain the structure of the cited text. That said, the cited text is a headline, title, etc, but it is not a noun phrase.
In The Cambridge Grammar of the English Language (Page 326) by H&P, the noun phrase (NP) is defined as follows:
Except in what we refer to as the fused-head construction (Two of them were broken; Many would disagree; It benefits the rich), NPs consist of a noun as head, alone or accompanied by one or more dependents.
The cited text does not consist of a noun as head, nor is it a fused-head construction per CaGEL (Page 1077, footnote):
Why (which has no counterpart in ·ever) appears freely in the interrogative construction, as in This is why I’m leaving, but is marginally possible in the pseudo-cleft: Why I’m leaving is that/because there’s no opportunity to use any initiative. It does not occur elsewhere in fused relatives.
Therefore, the cited text is not an NP but an interrogative clause. Now, a why interrogative clause is generally used as a main clause when why or why not is followed by a bare infinitival.
Why not go there?
??She asked why bother.
??She asked why not go there?
But when why is followed by a to-infinitival, it's the other way around:
This section tells you why to use page-level permissions. [from @Jon Purdy's answer]
??Why to use page-level permissions? [as a main clause]
CaGEL (Page 878) says that the infinitival interrogative is used exclusively for direction questions (cf. information questions). Here, by the infinitival interrogative I think CaGEL means to-infinitival interrogatives, but not bare-infinitival interrogatives. I personally think the marker to before infinitivals denotes a goal as in the preposition to, so it's understandable why direction questions seeking 'direction' requires to in the infinitival interrogative.
Now, the cited text from Google Support is a title, and titles can have many different forms including subordinate clauses. Therefore, it was legitimately used as a title showing a direction question that is normally only embedded in a matrix clause.
If to were omitted from the cited text, it would be an information question:
Why use page-level permissions?
This would be a title containing a main clause (with or without the question mark), because why followed by a bare infinitival may only be used as a main clause as shown above. And this could most likely be interpreted as questioning the validity of the decision to use page-level permissions over other types of permissions, thereby making the title sound less formal at the very least.
As others have noted, headlines are not always full sentences. Assuming that's not the issue, it is perfectly fine to have why followed by to (as part of an infinitive verb).
Perhaps you are more familiar with "how-to" titles, or similar phrases:
Well, why to is just as valid as how to, etc.