It's sometimes very handy to be able to get constant field values in encoders. Consider following composite type:

<composite name="Price">
    <type name="mantissa" primitiveType="int64"/>
    <type name="exponent" primitiveType="int8" presence="constant">-7</type>
</composite>

If you encode some price value given in double, you need to know the value of the exponent, but now there is no way to get it from an encoder.