There are languages with full inference that break this rule.
Moreover, this rule is more important for Rust than other languages because Rust makes a lot of constraints visible in function signatures.
But the most important purpose of the rule is communicating that this is a deliberate design decision and a desireable property of code. Unfortunately, there's an overwhelming lack of taste and knowledge when it comes to language design, often coming from the more academic types. The prevailing tasteless idea is that "more is better" and therefore "more type inference is better", so surely full type inference is just better than the "limited" inference Rust does! Bleh.
Moreover, this rule is more important for Rust than other languages because Rust makes a lot of constraints visible in function signatures.
But the most important purpose of the rule is communicating that this is a deliberate design decision and a desireable property of code. Unfortunately, there's an overwhelming lack of taste and knowledge when it comes to language design, often coming from the more academic types. The prevailing tasteless idea is that "more is better" and therefore "more type inference is better", so surely full type inference is just better than the "limited" inference Rust does! Bleh.