Hello there!
So this is my POV based on my experience. You don't NEED TO have a website for marketing. But the question that you should ask yourself is, how does your clients currently come from?

Let's say you are a doctor/lawyer. Most of this professionals get clients from referrals (previous clients) and people reach out to them when they need them.

If you had an accident and need a doctor, you probably won't go into Google and search for a doctor. You would go directly to the hospital to get treated.

So, if your product/service is not like that, then you should consider having a website if:

- Your selling process is complex (B2B or high value B2C sales like real estate or automotive, which could take from a month to a year or more).

- Your product/service requires a lot of education in order for prospects to buy. Your website must transform into the go-to place to find answers about the problem you're solving.

- There's a lot of competition; you need to differentiate and establish yourself in the market and the best way to do it is by producing content (blogs, articles, research) that should be published on your website. Having valuable information on your website will make people build trust with you, and could look out to you as a thought leader, if you are the one who provides answers.

Look at your website as an online salesperson 24/7. Should have al the information related to your product but more important, should educate visitors. If its a very good piece of content, you could get better rankings on Google, and could benefit from having higher rankings to get organic traffic (SEO).

SEO is the highest ROI channels, because eventually you won't be paying for that traffic, and when people come to your site by organic traffic, most likely they searched for your brand or something related to you; meaning, they have a need, so they have a high probability of conversion into a sale.

Hope this helps!

Best,
Felix