Like most birds, owls sing to attract a mate and defend their territories although perhaps not as musical as the songs of thrushes and warblers, the owl song functions in exactly the same way. The noises made by owls include the melodic trills of pygmy owls, the bell like chimes of the European Scops Owl, the classic hoot of the Tawny Owl (and it’s not so musical shriek), the strangulated screech of the Barn Owl and the deep resonant hoots of Eagle Owls.
In general the pitch and volume of the call reflect the size of the territory of the owl and thus how far the call has to travel. Of course, there are exceptions. Many forest species have loud calls for the size of their territories but this is because of the sound-deadening effect of the trees. The Flammulated Owl from the western United States is a similar size to the Western Screech Owl, which inhabits the same part of the world, but its territory is much larger and hence it has a loud deep call, more becoming of a much larger bird.
Diurnal species are far less vocal than nocturnal ones, being able to use visual displays to advertise for partners and defend their territories.
Don't forget, Owls don't breed very well when its Raining because it's Too Wet to Woo...