Why do most ship alternators generate 440V even though onboard appliance use 220V?
There is a balance to selection of switchboard voltage for ships. Higher voltage,  less current required to drive loads and thinner wires (less copper). But a lower voltage is a cheaper switchboard.
If you could get away with 24V or 48V, do it. No real need for switchboard. Which is the gist of the question! The problem is the electrical demand of ships is increasing. 
IEEE 45: Recommended Practice for Electrical Installations on Shipboard recommend 120V for ≤15kW and 230V or 240V for ≤100kW. Same components, motors, lights, etc. as used on land. 
IEEE45 recommend 450V for dual-voltage systems, with step-down transformers for lower power, land-compatible devices, lights, outlets, microwave, etc. 450V used for heavy loads to decrease currents in feeders and copper (pumps, fans). 450V switcboards would be the majority of diesel-propulsion ships.
450V generation and 440V utilization, ≤10V lost to wires.  Maximum capacity for a 450V generator is around 2.5MW.
As power requirements increase (electric-propulsion, reefers, LNG, passenger ships), higher voltage used to decrease \$I^2\ R\$ losses.  There are a number of voltages above 450V, but the main ones are 6.6kV for ships and 11kV for offshore platforms. 
The three main voltages for ships are 450V (≤5MW), 6,600V (≥7MW) and 690V. 
6,600V should be used for ≥5MW, but ship owners are reluctant to go there.  More expensive switchboard components and spares, specialized training, special PPE (rubber suits, gloves, boots, face shield), non-conductive stick to open/close circuit breakers. 
So between 5MW and 7MW is a grey area, where 690V is used with 500V switchgear (450V switchgear derated for 690V usage).
Both 6,600V and 690V use step-down transformers to get 450V and 120V. Real heavy loads on big switchboard (propulsion, thrusters, pumps, &gt;300kW loads), moderate on 450V (pumps) and light loads on 120V.
So at the core, higher voltages are used to decrease load current and save copper. But you must also factor in: availability of commercial products; total electrical load; and dangers of high voltages (&gt;1000V).
From high voltage marine safety training from Alpha Marine.