Ships of this period had masts for three main reasons. Firstly, yes, they were for propulsion. In the 1870s and 1880s, steam engines were relatively fuel inefficient. Ships could not carry sufficient coal for long voyages, and so needed to have sails for such cases. The Maine came at the tail end of the period in question, when steam engines and networks of coaling stations were sufficiently advanced to allow for ships to make typical voyages under sail. Even so, some contemporary or even later ships, like the British Condor class sloops, designed for duty in the Pacific (where coaling stations were rare), were built with sailing rigs. The second reason was communication. Even as late as WWI, the primary method of communication between warships in battle were signal flags. These had to be hoisted high up the masts, to allow the signals to be easily seen at a distance. Even when radio was introduced, the masts were still needed, as they provided somewhere to elevate the radio antennae, lengthening the range to which signals could be sent or received. Finally, the masts were important for gunnery control, and for finding targets. In the days before radar, the only way to find an enemy ship was to see it visually. Raising lookouts lengthened the visual horizon, lengthening the range at which another ship can be seen. The masts also provided a position, raised above smoke and blast from the funnels and guns, from which fire could be controlled.