My understanding (from reading arguments from language designers) is that curlies introduce error correction redundancy to a language. So if, for example, your editor decides to be stupid and mangle indentation of some copy-pasted snippet, the curlies provides the developer with semantic hints to deduce/reconstruct the original program.

This property is particularly important for readability of "embeddable" languages ("embeddable" in the sense of a language source code existing within another, for example JS within HTML script tags, or bash in a Dockerfile, or YAML string concatenation).