A synecdoche is a figure of speech where a part is used for the whole or where the whole represents a part. Why would someone want to use this figure of speech?
The juxtaposition of forms highlights the phrase or subject.
All variations on common language are intended to draw particular attention to one part or another. It is the little mind exercises that many find pleasurable and entertaining in reading.
Without these "turns of phrase" reading would be quite boring - particularly in fiction.
Some good examples and explanations of their use are here: