The sentence "it was necessary to explain" is correct and usable in written English.
You can use it any time you need to explain something that was important. For example, "I was concerned that my coworkers didn't understand the importance of the project, so it was necessary to explain the goals in greater detail."
Evidently, though, Comey felt it was necessary to explain to the public why the F.B.I. had made the unprecedented decision to launch a counterintelligence investigation into a major party's Presidential candidate.
When the Black Sea region was colonized by Greeks, it was first said to be the Amazon district, but when no Amazons were found there, it was necessary to explain what had become of them.
When the Black Sea region was colonized by Greeks, it was first said to be the Amazon district; but when no Amazons were found there, it was necessary to explain what became of them.
Girardi said he would probably talk with Nunez about what he can expect once Jeter returned, although he did not think it was necessary to explain to Nunez the long-established pecking order.
At that time the changes were so distressing that Bodin believed it was necessary to explain publicly the new circumstances in which France and the French found themselves.
To celebrate the Darwin bicentennial, it was necessary to explain how the measure of prehuman time had been increased by at least three orders of magnitude by Darwin's immediate predecessors, four orders by the man himself.
Now that those are established, it's necessary to explain how images of fractals are actually produced.
But since I am a woman it is necessary to explain why women have adopted revolutionary methods in order to win the rights of citizenship.
That it is necessary to explain the commons testifies to their neglect (despite the best efforts of political scientists such as the late Elinor Ostrom).
"Why should it be necessary to explain to 5- or 6-year-olds the meaning and implications of homophobia?" one educator asks in an editorial accompanying Ms. O'Grady's article.
Since the attempt would not only make him late, but also prove frustrat ing, it is necessary to explain that mother is going to tie the shoe laces today but that when he's a little older, he will be able to do it himself.
