The digital signature verifies the authenticity of the service or data. It is similar to signing a document to prove its authenticity. As an example, before buying anything from Amazon, we can check its digital certificate and will verify the authenticity of the website and confirm it's not a phishing website. Let's look into it further with a use case. In the previous scenario, Sean successfully decrypted the data he received from Chris. Now, Sean wants to send some confidential data back to Chris. It can be encrypted using the same method using Chris's public key. But the issue is Chris is not a part of the PKI setup, and he does not have a key pair. The only thing Chris needs to verify is that the sender is legitimate and it's the same user he claims to be. If Sean can certify it using the digital signature, and if Chris can verify it, the problem is solved:

Now in here, Sean encrypts the data using his private key. The only key it can be decrypted is by the public key of Sean. Chris already has this information. When Chris receives the data, he decrypts it using Sean's public key, and it confirms the sender is definitely Sean.