Both Silja and Viking ferries between Helsinki and Stockholm do overnight voyages and stay in the port for a day. They make a stop in Marienhamn, Åland in the middle of the night so they can sell tax free in their onboard stores, since Åland is outside the EU tax area, much like the English Channel islands. Most people actually going to Åland use daytime ferries which in Finland leave from Turku and Naantali.
With Helsinki-Stockholm ferries the cruise means that you spend a day in your destination city and take the same ship back in the evening. But the cruise is often much cheaper than one-way ticket, so people often buy a cruise also when travelling just one way. The difference is probably smaller or even other way around with higher class cabins, so chack the prices before booking.