Why am I completing a schedule c. I was paid for doing several product demonstrations for a company. They paid OOP costs. I was not an employee of the company

If you received a W-2 with the "statutory employee" box checked, or a 1099-MISC or if you were paid in cash, then as far as the IRS is concerned, you earned that money while self-employed and in business for yourself. Self-employment income is reported on SCH C as a part of your personal 1040 tax return.

If you are not an employee, then you are an independent contractor.  

An independent contractor reports their income on Schedule C.  In order to determine what type of income you had on schedule C it asks you if you materially participated. 

 If you were doing demonstrations to earn this money, then you will answer yes to did you materially participate.