Purpose of signing an .msixbundle file before uploading to the Store

Hi, I'm trying to figure out the process of uploading a Windows app created using .Net MAUI from a previous vendor. The process they described to me includes creating a single msixbundle file with one x64 and one x86 msix package inside it. So far so good. Then they use the Window Kit "signtool" application to sign it with a .pfx certificate before uploading the msixbundle file to the Store. I fail to understand why they need to sign it before uploading to the Store as Microsoft will sign it itself. They are not sure about why also. They also got the process handed over to them from the previous developers. Could be because they wanted to test the prod build locally once before uploading to the Store and that's why created the certificate for the same purpose? So that they can install that certificate in their system and test the prod msixbundle file? And then when they upload that signed msixbundle file to the Store, Microsoft just ignores that and signs it with it's own Store certificate?