Misunderstanding of Subnet directive in hosts files

[SVM]

Hello!

Could someone, please, explain to me the meaning of Subnet directive in 
configuration file of host, because I really don't understand this.

http://www.tinc-vpn.org/documentation/tinc_4.html#Configuration

"The subnet which this tinc daemon will serve."??? I assign an IP from 
apropriate subnet to interface, what does it mean "will serve"?

I've read documentation, but it doesn't make sense to me and my 
configuration seems to work fine without any Subnet directive specified.

In which case should I use Subnet?

I have simple tinc.conf:

Name = aaa
Interface = tap0
Mode = switch
ConnectTo = zzz

And /etc/tinc/netone/hosts/zzz:

Address = <ip-addr>

-----BEGIN RSA PUBLIC KEY-----
-- my public key here --
-----END RSA PUBLIC KEY-----


Many thanks in advance.