I don't understand why people find this so difficult to grasp. No one has difficulty understanding that, for example, A sharp and B flat are different ways of describing the same note, so why the difficulty with B# and C?

It's important if you want to understand music because some key signatures show B#. Whether you need to use this knowledge is another matter - if you only play in "folk keys" then you won't use it. But understanding it helps you to understand how scales work and how notes are described.