[danshapiro]

IANAL (just the OP) but - imagine you invent a self driving car, and you'd like to patent it. Your lawyer tells you not to read prior art, because you don't want to be liable for willful infringement. Also you've been living in a cave, so you don't really know what prior art there is out there. You write claims like this:

Independent claim: Self driving car. (You really hope you get this, but it's so broad, you probably won't).

First dependent claim: The previous claim, where in the self driving mechanism is an actuator applied to a gas pedal. (Now you're going after the specific way you accomplished the broad problem - hopefully, even if someone's patented a self driving car before, they did it some other way, so you can protect your preferred category of solutions, and force competitors to take a different approach).

Second dependent claim: the first dependent claim, where the actuator is a brick. (If all else fails, you've at least protected the precise mechanism that you used - not enough to keep out a competitor, but at least enough to preserve your unique solution).

The patent office will often strike the independent claims but leave some dependent claims around, so you craft them strategically to get at least some protection.

[Tomte]

But if the patent office doesn't accept your independent claim, one of the dependent claims must become a new independent claim, right?

So it's simply for the process of getting a patent (and some cost issues with registration)? After the patent has been granted they are truly unnecessary?

[fncypants]

You may have come up with what you think is a great new idea and several variations that are also useful. You claim the broadest one as an independent claim. And the refinements are dependent claims. The patent office may agree, and issues all claims.

If, later it turns out the big idea was not new, but the refinements are, then you may find yourself with an invalid independent claim but valid dependent claims (which are treated as though they stand on their own). Although you could have filed the dependent claim as independent just the same, it saves paper and cognitive time analyzing it, which is a good thing. And for that reason patent fees are more expensive if you add more independent claims rather than dependent.