Mainly to prevent tampering and cracking. For example, the program you write includes 1 exe and several dlls. EXES took a lot of effort, and it has your company's logo on it. Now a hacker decompiles a dll and finds that it is easy to modify a dll, such as a dll with a registration code, which leads to bypassing the verification code. Another hacker changed the good dll into the function of deleting all files on the disk, causing users to blacklist your company.

If you sign it, you can guarantee that exe can only call its own dll. If they tamper with your dll, exe will report an error and the dll cannot be found.