Vastly more power and flexibility, with a highly usable Web interface (LUCI)
and console access.
Regular updates.
Excellent documentation.
https://openwrt.org/docs/start
And a whole set of packed applications and tools. There are about 6,000 packages in total, ranging from device-specific and kernel support to advanced applications such as media servers. Link below is just the larger apps.
https://openwrt.org/docs/guide-user/services/start
Re-romming my DSL modem (500 Mhz dual-core CPU, 64 MB RAM, 8 MB Flash), as advantages over stock vendor firmware, I get:
- SSH access, rather than periodically-enabled telnet
- adblock
- remote logging capabilities.
- performance and activity monitoring.
- consistent interface with my router (also running OpenWRT).
- Full-featured shell tools rathee than barebones Busybox versions, if I like.
- Remote filesystems / additional storage.
Depending on your device(s) and capabilities, your modem, router, or other hardware can serve as a home server: NAS, UuuNextCloud, Webserver, VOIP services, media server, PXEBoot (useful for testing images/deployments), guest network(s), VPN, proxy servers, email, mesh networks, messaging, captive portals, and far more.