You have a 50GB limit/month from your internet provider, after which your internet will be capped at 256kbps. You have 5 users and you can put a 10GB limit/month everyone on your network, when their limit is up, you can decide to slow down their internet or cut it off altogether. This way, everyone are happy and can get their fair share.
You have 25GB (peak) + 50GB (off-peak) limit/month from your internet provider, after which your internet will be capped at 256kbps. You have 5 users and you can put a 5GB limit (peak) and 10GB limit (offpeak) /month for everyone on your network, when their limit is up, you can decide to slow down their internet or cut it off altogether. This way, everyone are happy and can get their fair share.
You uses 3G/satellite internet connection with 5GB limit/month, after which you will be charged a fee per MB. You can cut off the internet when the limit is almost reached, e.g. 4.8GB.
Or if you are sharing this internet with others and they are willing to pay for excess data, you can check the usage of every devices connected and then divide the excess fee proportionly among all everyone on your network.
You want to setup a public WIFI access point and restrict the internet bandwidth or put a quota on every connected devices.
You want to restrict the amount of time your kids spend online. You can block internet access on specific devices during specified period of time without affecting others.
You are suspicious about your kids/employee's online behaviour. You can specifically monitor your kids/employee's device and see what web sites they visit.
Note: url monitoring only works for unencrypted traffic, i.e. those web sites using https will not work.
You want to block access to specific web site/applications during business hours, so your employee don't spend paid time browsing ebay or chat over skype.
You want to share files stored on USB drive using ftp/smb/DLNA across your LAN and access your files remotely.
Note: DLNA file sharing will require installation of Openwrt applications using command line (SSH).
You want to access your LAN's shared files or use your home internet remotely (e.g. overseas) as if you are at home. This way, you will have access to region restricted internet content which you can't access from overseas, such as region restricted audio/video streaming/subscription web sites, country restricted web sites, etc.
You can do this using VPN connection or SSH tunnel. You can configure your router either as the server or client. You can have two devices at two locations to bridge your two remote network, one acting as server and the other as client.
You want to setup a separate Wi-Fi network for your guest in addition to your own family Wi-Fi network. You can place quota/bandwidth limit for all your guests while having full speed access for your family. If you have quota/bandwidth limit in place, you could leave your guest Wi-Fi network open without password too.
Note: If you do not set a password for your guest network and did not block access to your router from the guest network, you will inadvertly leave your router open and vulnerable for hacking.
You have unlimited internet access but someone is always downloading heavily or use bittorrent and slowing down the internet so badly that you can't hold VOIP/Skype/Gtalk/Facetime/etc conversation. You can use QoS to prioritize your VOIP/Skype/Gtalk/Facetime/etc traffic over others, ensuring you can enjoy good quality conversations.
You can find additional examples of common sceranios here.