I have read, somewhere or other, that when Avast locates infections and removes them to the Chest, it is better to leave them there for a time than to delete them immediately. To me, this defies logic.
Can someone explain, please?
The adage 'first do no harm' e.g. don't delete as now you have zero options left.
So sending to the chest and then deleting would be the same as deleting it in the first place. As has been mentioned already false positives are a fact of life in security applications (though many simply won't admit to this). Depending on what the malware name of the detection was, it could be Heuristic or generic detections and those are more prone to false positive.
For example the avast Win32:Trojan-gen is generic signature (the -gen at the end of the malware name), so that is trying to catch multiple variants of the same type of malware and is a fine balance between detecting a new variant and detecting something valid as infected.
So you should leave detections in the chest for a few weeks before a) scanning it again in the chest and b) deleting it if it is still detected. During this time (2-3 weeks) you should be watching out for any adverse effect from having sent the file to the chest, error messages, missing file, etc...