I always use the example of building a $1 million dollar website. You can spend 100% of that on the website (design, development, copy, images, video productions, etc. etc.) but when it launches no one will visit it without any marketing. If you split that budget 50/50 between the website it may not look as nice but you'll have WAY more people coming to the website when it launches.
For a business, at the end of the day, is it more important that people find it, or that it is very pretty? You can always improve how a site looks, but it's important to invest in SEO/marketing early so that it can slowly build up over time.
This is an oversimplification and it assumes a brand new domain, etc. but that's why I love SEO. I'm originally a web developer and I realized that when clients asked me to 'make me a website', I eventually realized it that they meant, 'i want to make money online'. Then I started having less conflicts with my clients and I could choose who I wanted or didn't want to work with.
Marketing gives power to the client, and to me. Win-win.