There is a plan.
Sometimes you simply want to say something exists. To be means to exist (among a lot of other things), but you can't say:
A plan is.
and be easily understood. This is because to be is a copular verb, and used that way so often that, while the above sentence is technically correct, most listeners/readers will be expecting you to say A plan is X and will be waiting for the X.
So you can say There is X to say that X exists, when you don't want to say X is Y.
Why not use the word exists? You can, but for some reason it tends to sound weird in everyday speech except when talking about the nature of something. There is X is more idiomatic in everyday speech.
There is X doesn't automatically mean X is good. For example, there is danger, let's turn back wouldn't make "danger" a good thing.