¿Qué significa That way you won't have needlessly cleared your calendar only to be left standing at the curb. ?

See a translation

I am not sure what the context is, but

The person saying the sentence wants to make sure that the listener does not make time for, or plan for, an event that does not happen.

for example:
You and a Friend plan to go to dinner.
You arrive at the restaurant, but your friend does not show

You have made plans/appeared for/committed to an event, but it did not happen.

You have made no other plans ("cleared your calendar")
and gone there (the "curb", but not really)
but no one came. (you were "left alone" there)
because no one came, you did not need to come. ("needlessly")

the speaker wants this to not happen, so they will do some action to make sure you are not "left standing at the curb"

this may be a big answer, I hope it helps