Hey koisita,
That's a self signed certificate to encrypt the launcher's traffic. It's so we can use something called http/2 protocol to send you the patch files you need. We didn't want the http/2 protocol for the encryption, it's just a quirk of how the launcher is written. Http/2 gives us some configuration options that makes the download speeds a lot faster. I THINK (not sure) that the way I set it up, you can choose not to accept the certificate if you're really worried about it, but it'll slow your traffic significantly. There's more info here:
https://www.cloudflare[DOT]com/learning/performance/http2-vs-http1.1/
Thanks for the help!