Maybe I wasn't clear -- most houses have a single panel with multiple circuit breakers. Each breaker services a separate circuit. Each circuit represents a single run (hot, neutral and ground) from the panel to the destination. Only really old houses that haven't been upgraded use fuses. The panel itself has two buses, one for each 110v leg, a common neutral bus and a ground bus, a main breaker which controls the entire panel. The legs aren't switchable.
The fuzzy part is the "destination". Since each circuit is usually breakered for 15A it can support more than a single outlet or light fixture. When an electrician wires the circuits, they commonly will run the cable from the panel to, say, a socket and then feed off the socket's secondary terminals (or via wirenut splice) to another close by socket or light fixture. And "close by" doesn't mean "in the same room" or "the same type of connection" -- if there was another socket on the other side of the wall installed at the same time, because the distance is close and it's easy for the electrician they will often connect that socket to the same circuit as the first one.
What you end up with is basically a parallel circuit of devices (light fixtures and sockets) that are close together "as the crow flies" but not necessarily in the same room or a common type of connection (socket or light fixture). This is especially true of remodels or small-scale room re-dos or where people have wanted additional outlets and rather than rip out a bunch of drywall, they will tap from the closest place they can.
Large draw devices (central AC, electric stove, hardwired electric heaters or furnaces) will have their own, dedicated high-current breaker at the panel and a dedicated run from the panel to the device. It used to be allowed years ago to even tap off one leg of a 240v circuit to get a 110v, but codes are tighter and these days a new install will require a dedicated run.
So what you usually end up with a single panel with a handful of dedicated breakers for high-current and 240v devices and then a bunch of other breakers which control the outlets and fixtures in a specific area, but which may also control other outlets/fixtures "nearby" often with no logic other than what made sense for the electrician when the wiring was done.
What I think is needed is much stricter cabling standards and structured panels. One panel should control lighting with a dedicated breaker for each room and ONLY fixtures IN THAT ROOM connected to it. Another panel should control general purpose outlets by room. A third panel should control high load devices (electric stove, central AC, other major electric appliances). A fourth panel for "mandatory devices" you would always want priority given to, such as refrigerators, furnace blower motors for gas or oil furnances, etc, perhaps a few "emergency" outlets for computers or USB chargers).
With a properly structured panel system a transfer switch could then feed the lighting and mandatory device panels but leave the high power devices and general outlets off until mains power is restored without risking overload of the backup source or from vampire loads connected to standard outlets that aren't critical during a power outage. Of course this is a lot more expensive to install because you need much more cabling, more panels and more wall space to place the panels.
A better option, IMHO, would be a smart panel or maybe even smart breakers which could be individually controlled so that you can assign the basic panel "values" and determine which ones to run under specific non-grid scenarios. Such a control system tied into the backup system's monitoring and capacity could then switch off or on circuits as power was available or as loads were brought up/down.