It can matter if you can trust them to do the proper thing, i.e. if you assume they are not a malicious entity. In this case, checking the server source code ca

It can matter if you can trust them to do the proper thing, i.e. if you assume they are not a malicious entity. In this case, checking the server source code can give experts insights about possible security risks. If you assume they are malicious, (a) I wouldn't use their product in the first point, and (b) of course they can do whatever they want independently from the published code.