1. I'll wear whatever they give me.
2. I'll wear just kite string if they want.
#1 is abstract, because categories are abstract. You cannot picture in your head 'clothing' or 'whatever clothing they give her'.
Meanwhile, an instance of the category is concrete. It's something you can visualize and imagine.
So, the most common – and best – reason for using this synecdoche is for vividness. It's easy to imagine 'a circle' and impossible to imagine 'a shape'; it's relatively easy to imagine 'a long knife' and impossible to imagine 'a weapon.' Voluneering at the County Hospital, as abstract as that might be, is more concrete and vivid than charity work.
Use synecdoche for vividness.
Also, a vivid instance can be chosen. Kite string! The County Hospital is not very vivid, but I suspect Chandler didn't have a lot to work with and did the best he could.
So, how does this work? I'm editing, and I discover a sentence that . . . well, at first I don't like it as much as I should, so I stop and think about it. Suppose part of my sentence seems too abstract.
1. I walk around the room in my bare feet, looking for any food that might have been left on a table or on the floor.
Food is abstract; that story wanted kite strings. So I substitute an instance of the category:
2. I walk around the room in my bare feet, looking for a broken piece of pretzel that might have been left on a table or on the floor.
That image crackles with vividness. The broken piece of pretzel had gained permanent residence in that sentence and could not be removed by anything except a direct meteor strike.
The actual communication? Unfortunately, not as good as I want. The reader would at first see this as being about pretzels. That's not too suprising – the sentence does after all does say pretzels. The reader would figure everything out eventually, but I am no fan of "eventually."
Would the reader quickly figure it out? If the answer was yes, I would have left the sentence the way it was. But I decided the answer was "probably not."
So, if (1) you use an instance of the category but (2) you want the reader to understand you're talking about the whole category, how do you accomplish this?