Not mandatory! But I would suggest you to make it mandatory, if you expect relevant visitors to visit your business website online. I agree, free SEO is slow, but steady. You can also try google adwords, email marketing, facebook marketing, free classifieds online and more. Hope you can conclude now. Good luck for your query.
From the various responses I have read, SEO doesn't seem mandatory, but then I have a different opinion...when you are in the online bizz...your presence matters...and before that you need to get discovered. SEO is an age old technique to be discovered, hence I would not discourage you from doing it. As long as you know what you are doing in SEO and how effective it is in terms of your Return on Investment, I think I will recommend it. Besides this I would like to lay stress on the fact that every digital marketing technique has it's role in the overall process. You need to maintain that balance, so thumbs up for SEO.
If you want only traffic so you can use social media and that's it for you. But if you want to get some organic searches so you need to hire a SEO guy.