They went there in the afternoon to reserve a good spot.'
'in the afternoon' modifies 'went' and is more appropriately placed after the verb. At the end of the sentence, it still has the same meaning, but seems unnatural...and makes the reader wonder if you didn't intend 'for the afternoon':
They went there to reserve a good spot for the afternoon.'
Now, it means they went at some earlier time to reserve the spot in readiness for the afternoon.
So - what would this mean: "They went there for the afternoon to reserve a good spot."
Student or Learner