1) providing basic networking (default gateway, connection to upstream provider)
2) basic security: firewalling + natting
3) basic network daemons, and "daemons": DNS, DHCP, IPv6 autoconf, and a nice new one: 802.11s ("network mesh": keep wifi at 5 bars by kicking people off their access point if another access point can provide a better connection)
4) more advanced networking stuff: VPN (l3vpn, l2ptp, l2ptmp, mesh vpn), cloud connections/vpns, WAN connections, redundancy (downstream and upstream, l2 and l3, ...)
Longer answer:
1) providing basic networking (default gateway, connection to upstream provider)
2) basic security: firewalling + natting
3) basic network daemons, and "daemons": DNS, DHCP, IPv6 autoconf, and a nice new one: 802.11s ("network mesh": keep wifi at 5 bars by kicking people off their access point if another access point can provide a better connection)
4) more advanced networking stuff: VPN (l3vpn, l2ptp, l2ptmp, mesh vpn), cloud connections/vpns, WAN connections, redundancy (downstream and upstream, l2 and l3, ...)
5) more advanced security stuff: 802.1x, stateful firewalls, inspecting firewalls, IDS, ...
6) server networking stuff: proxy, reverse proxy, statefull firewalling, load balancing, ...
7) more advanced daemons (on network devices it's mostly proxying for redundancy and/or load balancing): File sharing, SMTP, WebDAV, Active Directory, payment protocols, ...
Now a lot of these OpenWRT can't provide, but you'd be surprised and their support for new things goes up every year.