I've also noticed this accumulation of resource files in that folder. I've researched a little and found a possible explanation here: https://dnntracker.atlassian.net/browse/DNN-9461?focusedCommentId=124417&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel
But I've also found the request to clean that folder up at https://github.com/dnnsoftware/Dnn.Platform/issues/2779. This has been waiting for almost one year now, for someone to step forward and implement a simple procedure for cleaning it. Maybe it's a good moment now, when sites import/export proccess is being reviewed, isn't it?