A sentence can keep growing

The image networks from earlier modules received a fixed grid. Before an image arrived, the network already had a place for every pixel.

A sentence does not come in one fixed size.

Stop. has one word.

The dog ran home because it heard the gate open. has ten.

A paragraph may have hundreds. If a machine prepares ten word spaces, word eleven has nowhere to go. It could prepare hundreds, but some longer passage would still cross the limit.

The machine needs a way to handle however many words arrive.