It’s not functioning as a thank you, it’s honestly just an indication that you’re willing to take an extra 5 minutes to do something when the stakes are high.
If you really want the job, how you treat that conversation might be similar to how you treat a client once you’re hired. If you don’t really want the job, or you really don’t want to send a thank you, that’s fine, but with 5 applicants to choose from, wouldn’t they opt for the candidate who put in the best/most work?
I’m not saying it isn’t annoying and transactional and a lot to ask, but as someone who’s applied for hundreds of jobs, the reason to do it still feels clear.
I’ve always assumed you pay extra because multiple people have to carry the bag around after you check it, and that’s harder/more dangerous at higher weights.
In warehouses, you gotta go get your lift belt and often a partner if something is over a certain weight, and you aren’t covered by workman’s comp if you just try to do it quickly without those, so it’s a serious hassle.