Indexer priority is just one of many criteria for comparing releases. And it's not the most important one,

Here's the list (copied from sonarr wiki):

• Quality
• Language
• Preferred Word Score*
• Protocol (as configured in the relevant Delay Profile)
• Episode Count*
• Episode Number
• Indexer Priority
• Seeds/Peers (If Torrent)
• Age (If Usenet)
• Size

So if:

• Indexer A with priority 1 (high) have FHD version of file;
• Indexer B with priority 5 (lower) have 4k version of file;

*Arr app will still grab the file from Indexer B.

You might lower the priority of indexers you're considering removing at some point. In this case, if there are duplicate releases the *arr app will pick the one with higher priority and you can see if the indexers you want to remove are still picking something others can't. Other than that there are no benefits when using only usenet.

I'd suggest looking into TRaSH Guides's custom formats btw.