Can message length be useful information?

Suppose a packet is encrypted and sent via an insecure channel so that it is intercepted by a malicious third party as well as the intended recipient. As long as a suitable encryption scheme is used, the message should be (practically) uncrackable.

However, assuming that encryption preserves message length to a certain degree, the third party will gain some info about the size of the message. Is there any context in which knowing only a message’s length could be useful to a hacker? If so, what are some examples?