1. Package source hacking. Say an attacker gets access to a package source's storage and inserts malware into popular packages. Now your project contains malware. Your package manager can detect tampering if the packages are signed.
2. Dependency confusion attacks. Say my project downloads packages from the public source as well as my company's private source. An attacker realizes my company has a private package named `private-foo` and uploads a malicious package with the same name to the public source. Now my projects contains malware as it occasionally downloads the malicious package instead of my company's private package. Your package manager can detect packages from an unexpected author if they are signed.
I can provide more examples if you'd like. None of these are hypothetical, all of them have happened already.
Just want to point out that bundler solves this problem (and many others). It pins gem versions in Gemfile.lock and it supports explicit source locations (like git repositories) for downloading gems.