You can say for one thing when you are explaining a statement or answering a question, to suggest that you are not giving the whole explanation or answer, and that there are other points that you could add to it.
She was an excellent colleague. For one thing, she was very reliable.
She was unable to sell it, because for one thing its size was awkward.
'How have the sanctions affected your life in Belgrade?'—'Well, for one thing, we already have shortages.'