Online ads like GoogleAds, Ad Remarketing, AdRoll and Facebook Ads are a great way to get your site in front of targeted customers, but it can be expensive. Ad systems like these are a one and & type of interaction; meaning if you pay for your results to come up first you will be paying each time a user clicks on that link. If you turn off the ad those links will go away. To keep those clicks coming, you have to keep paying for them.
With SEO, you have the ability to build content that is loved by search engines and delivered for free by the search engines. Investing in SEO early on will help your website be found more often and at a lower cost over time. Just like any investment, SEO takes time to build, but when you invest early the rewards will be bigger in the end!
For most new websites, investing in ads early on while still working on SEO is a great approach. Over time you should be able to limit your ads and without hurting your website visits because your SEO of taking hold.