Often business or other ideas come at the most inconvenient times.
When you are working, walking, doing something completely unrelated. Or you might see something that gives you an idea.
Got nothing to write with? Dictate it into your phone, make notes on your phone or ring yourself and leave a message.
Chances are that you will forget 98.8 percent of them more or less straight away and another 1 percent (the one you really want to remember!) over the remainder of the day.
What does that leave you with?