I read this page about DNS caching and I really wonder what the advantage is TTL-wise like it's written there.

The (windows) DNS client caches the results with TTL it receives from the recursing DNS server. Could be a local DNS server (like I have) or DNS servers from the ISP or an open DNS server. In all cases TTL is used and queries on the authorative DNS servers are kept to a minimum.
With DNS caching there are less queries from Smartermail to it's recursive DNS server set at Settings > General > Server Info

But I am running a local DNS server, is there any advantage using DNS Caching in Smartermail with a local DNS server?

Where does Smartermail caches DNS queries. Windows DNS server caches in memory. When SMartermail caches on file, than Smartermail caching probably is slower.