OpenVPN server can act as simple-tunnel, P2P, Server, Server-Bridged. Each has different networking/routing approach. No matter which type you choose, you can always make your services available to vpn-clients. It is matter of forwarding rules and traffic control rules on your firewall.
Main reason for changing default lan network is because that is pretty much generic and widely use. Client ip might be from same range as your local lan (so routing later on might get fuzzy, especially when your NAS ip is also present on client home network … like he will have his own nas on very same ip 
)
Check those first :
https://openvpn.net/index.php/open-source/documentation/howto.html#vpntype
https://openvpn.net/index.php/open-source/documentation/howto.html#numbering
I put some links to howto/guides here : Using Turris as a VPN client for most outgoing traffic I am still using 192.168.1.0 (not changed yet … i am still playing with openvpn configuration
Here is my last used config setup: ACCEPT vs DNAT (port forwarding) firewall rules