to be (doing something)
Some sentence structures require you to use an infinitive ("to" + a verb). For example:
I want to leave.
She told me to call her.
When you need to use an infinitive, but you also want to talk about an ongoing action, use the form "to be ___ing"):
I told you to be waiting for me when I arrived.
On a day like this, I don't want to be sitting in a dark office. I want to be laying out in the sun.