The short sentence "Everyone waited" has the effect of increasing suspense and tension.

In Roald Dahl's Matilda, the chapter called "The Weekly Test" begins with the following paragraph:

At two o'clock sharp the class assembled, including Miss Honey who noted that the jug of water and the glass were in the proper place. Then she took up a position standing right at the back. Everyone waited. Suddenly in marched the gigantic figure of the Headmistress in her belted smock and green breeches.

Of the four sentences that make up this paragraph, the third is by far the shortest, and it is not strictly necessary to the reader's understanding. However, it contributes to the reader's sense that something significant is about to happen. Miss Trunchbull's arrival, when it comes, is unwelcome and frightening, but it is not unexpected. It is for this that everyone has been waiting.

The sentence is noncommittal enough to build up tension without revealing anything about what is to take place. Dahl could have written something like "Everyone waited, trembling in abject terror at the frightening prospect before them." This, however, would have been too extreme and led to an anticlimax at the end of the paragraph. The sense that everyone is waiting for something about which the reader is, as yet, in the dark increases the sinister feeling of suspense in the passage.