netifd is an RPC-capable daemon written in C for better access to kernel APIs with the ability to listen on netlink events.
Netifd has replaced the old OpenWrt-network configuration scripts, the actual scripts that configured the network, e.g.
netifd is intended to stay compatible with the existing format of
LEDE network configuration, the only exceptions being rare special cases like
aliases or the overlay variables in
/var/state (though even most of those can be easily emulated).
One thing that
netifd does much better then old OpenWrt-network configuration scripts is handling configuration changes. With
netfid, when the file
/etc/config/network changes, you no longer have to restart all interfaces.
/etc/init.d/network reload. This will issue an
netifd, telling it to figure out the difference between runtime state and the new config and apply only that. This works on a per-interface level, even with protocol handlers written as shell scripts.
It boils down to the fact that the current network and interface setup mechanisms (via network configuration scripts) are rather constrained and inflexible:
Netifd will be able to manage even complex interface configurations with a mix of bonding, vlans, bridges, etc. and handle the dependencies
between interfaces properly - and of course all that without adding unnecessary bloat.
AFAIK there are no alternatives to netifd, e.g. connman seems to be centered around one specific use case only: having a mobile device access the internet through multiple connections.