The server key is the key that DNScrypt uses in order to create a secure connection with the domain name servers. Periodically refreshing it is a way to make sure it has not been tampered with or revoked.
If hackers were able to forge a security certificate, they could use it for what's called a "man in the middle attack," tricking your computer into talking to their server rather than the legitimate DNS server, then passing on your computer's traffic to the real server while they eavesdrop on what's going on.
Should something like that happen--which is extraordinarily unlikely--the certificate that your computer has a copy of would be revoked and a new security certificate issued. By fetching a new certificate periodically, your computer can be sure that the certificate it has is valid and hasn't been compromised or revoked.
Having said that...once an hour is an excess of caution.
