I started a series of modal advantages a number of days ago. This entry is kicking those back off. The last entries covered light rail for the light rail fan kids, streetcars for the streetcar fan kids, and now it is down to the buses. Without further adou, I'll jump right into these work horses of transit authorities across the world.
Legitimate Reason #1 – Existing Infrastructure
Because Governments have spent the last 100 years pouring trillions of dollars into roadways across the planet (the the adoring love of every automobile manufacturer), it doesn't need to be mentioned but I will, there are millions of miles of roadways on the planet. Because of this buses often are the least expensive by an order of magnitude in capital startup costs. Rarely are the costs of the roads ever connected to buses. So with a mere couple million dollars a city can have a bus line with a decent frequency and amiable service levels.
Legitimate Reason #2 – Flexible Remote Routes
Buses can carry a reasonable amount of people, and make connecting remote locations more economically reasonable than running any sort of rail to remote locations. The higher long range cost is overshadowed when it comes to lower ridership routes, where a minimal fluctuating system is needed. The only tangible mode that can operate on routes that are distant and highly fluctuating are buses, and sometimes in really low ridership routes, vans. But I'm sticking to buses right now, since most countries won't utilize vans as a mode of transport within a transit authority.
Legitimate Reason #3 – EXTREME Flexibility
Again, because of the vast investment in roads, if one is blocked, buses can go around. If there is a wreck, buses can go an alternate route. If a bridge is out, buses can go over another bridge. If there is a car parked a couple of feet too far into the road, a bus can simply go around. What happens if ridership drops to unreasonable levels, you can cancel the bus line and the road can still be used by autos, bikes, and even foot traffic.
Legitimate Reason #4 – Choices in Vehicles
Buses come in multiple options. 40' buses, extended buses, intercity buses, urban buses, luxery buses, etc. In today's day and age, if you want choice in options, the bus provides that in droves. Since infrastructure costs are often mitigated by general budgets and taxes, buses are seen as an efficient mode of transport and the market centers more on this mode than any other.
Legitimate Reason #5 – Acting Starters for High Volume Routes
Buses can also play a relatively new role, as a primary corridor mode on dedicated right of way. This slightly newer technique, per the light rail transit nomenclature is referred to as bus rapid transit, or BRT. BRT has been used as a starter system predating a light rail line, handling the initial years when an area is unsure of the need for higher volume light rail capacity, but knowing a higher volume dedicated corridor is needed. In this service, buses often range from 40', to the more common 60' buses. BRT is an inexpensive way to create a dedicated right of way when the future is uncertain, because this leaves the corridor in a position to transition to road only use by other vehicles without the rail investment being made that usually requires greater capital funding usually ranging 2-5x as much.
So that's the list I have, which I'm sure there are readers who could add another dozen points. These however, are the main, obvious and outstanding points of advantage for any bus system.