Three or more support points will not share the load evenly (unless they are hinged in a non-rigid whiffle tree or similar).
A beam supported only in the middle will sag at the ends, making a similar shape but upside down.
Supporting a uniform beam at the Airy points produces zero angular deflection of the ends.
They are much less convenient to use than end standards[5][6] but, when the marks are placed on the neutral plane of the beam, allow greater accuracy.
To support a line standard, one wishes to minimise the linear, rather than angular, motion of the ends.