My apartment in Queens is close to a subway line, but I also have another subway line and a Long Island Railroad station a short walk away. There are buses as well, that can take me to Manhattan, Brooklyn or other parts of Queens. I can also get to many destinations by bike, or even on foot if I walk long enough. We also have taxis and gypsy cabs and car rental agencies.
This redundancy is important for many reasons. It can allow the system the capacity to deal with heavy demand. If one part of the network breaks down, I can use another. If the MTA needs to upgrade the signals on one line, I can use another. When one union went on strike, I was able to use the buses driven by workers from another union. When massive power failures shut down the subway and the resulting chaos clogged the roads and swamped the buses, I was able to walk home.
Streets in a grid system have redundancy built in to them, which is good for buses, taxis, bikes and pedestrians, but it's only good for car drivers and passengers as long as they have a system to fall back on if their vehicles break down.
When planning for new transportation systems or expanding existing systems, it's important to build in redundancy. In the short run it may seem better to add capacity to existing lines or streets, and in so cases it is. But when the existing lines don't have redundancy, it can be better in the long run to add another line or system.
There are several issues I've come across that relate to redundancy, so I'll be referencing this post in the future. I wanted to lay the groundwork here: redundancy is usually good in the transit world.