Why is seo important for business?

Because it's the one channel where people come to you already looking for what you sell, and you don't pay per click. Ads stop the day you stop paying, rankings don't. On the core update bit, some movement after an update is normal. Give it a couple of weeks before changing things. If it's still down, compare which pages lost traffic in Search Console and check whether the pages now ranking answer the query better than yours.