It's optional. But you need an EIN if you pay employees. You might want to get an EIN so you don't have to give out your ssn like if you pay any sub contractors and need to issue them a 1099NEC. Or if someone pays you over 600 and wants to give you a 1099NEC to report the income.
Here is some IRS reading material……
IRS information on Self Employment
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Self-Employed-Individuals-Tax-Center
Pulication 334, Tax Guide for Small Business
http://www.irs.gov/pub/irs-pdf/p334.pdf
Publication 535 Business Expenses
http://www.irs.gov/pub/irs-pdf/p535.pdf
Publication 463 Travel, Gift, and Car Expenses
https://www.irs.gov/pub/irs-pdf/p463.pdf
