It’s probably more of a scale thing, going a conversation server side need CPU time, if it can be done prior to upload then server time is reduced. I think a lot of websites do client side processing so they can do more requests per server instance.

The point of having a high entropy password is to protect against hackers brute forcing a leaked database of hashes.

I don’t think you need to worry about that in this case, the special character restriction suggests to me that they don’t hash it.