What are the dangers of uninitialised variables?

These variables could contain any value if you don't initialize them and reading them in an uninitialized stated is undefined behavior. (except if they are zero initalized)

And if you forgot to initialize one of them, and reading from it by accident results in the value you expect it should have on your current system configuration (due to undefined behavior), then your program might behave unpredictable/unexpected after a system update, on a different system or when you do changes in your code.

And these kinds of errors are hard to debug. So even if you set them at runtime it is suggested to initialize them to known values so that you have a controlled environment with predictable behavior.

There are a few exceptions, e.g. if you set the variable right after you declared it and you can't set it directly, like if you set its value using a streaming operator.