The importance of signalling for controlling train movements was recognised very early. Railways cannot be operated safely without some form of control as the vehicles, unlike those on a road, cannot get out of the way of others coming in the opposite direction either quickly or easily, nor can they be stopped as readily.
A model railway without signals lacks realism. Signals are, by their nature, very visible things and their absence is immediately apparent. Unless the area being modelled is an industrial siding complex where trains are under the control of a shunter, (and even here there could be an exit signal on the lead to the main line) signals are necessary to give an authentic appearance. Not all modellers choose to make their signals work, but even static signals should be located correctly on the layout.
Many modellers find that if signals are provided and operated correctly, the overall air of realism is considerably improved. On large layouts with several stations a proper service cannot be operated without some form of control. Although intercom sets allow operators to speak to each other, there is a strong argument for following full sized practice and communicating via the signalling system. Until well into BR days, this would have been based on block systems and bells. Modern computer controls of train systems have largely replaced this, and some layouts have also taken a similar route.
Additional articles on, for example, block working and interlocking would be very welcome.
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