You can go back into a time before computers and use the most famous examples of how bad RNGs endanger crypto: One Time Pads (OTP). This is basically just a XOR of your message with your random numbers. If your random numbers are bad they leave patterns in the XOR-ed message that can be statistically analyzed and potentially decrypted.
Bad PRNGs repeat early and have predictable patterns, as such they form a special attack vector.
Whether that is really an issue for any given application, depends on the application (e.g. how shortlived the crypto is and how bad decryption acter the fact is). But if you are asking that question it likely means you don't know enough knowledge to roll your own crypto unless you wanna be in a world of pain.
Just because PRNGs appear unpredictable to you does not mean they aren't.
If you really wanna go the route, at least add a salt, so something like
Bad PRNGs repeat early and have predictable patterns, as such they form a special attack vector.
Whether that is really an issue for any given application, depends on the application (e.g. how shortlived the crypto is and how bad decryption acter the fact is). But if you are asking that question it likely means you don't know enough knowledge to roll your own crypto unless you wanna be in a world of pain.
Just because PRNGs appear unpredictable to you does not mean they aren't.
If you really wanna go the route, at least add a salt, so something like