Let's break down the practical facets. For non-fiction, platform is non-negotiable; it's your credibility. For fiction, it's a 'nice-to-have' that's rapidly becoming a 'must-have.' Agents evaluate reach, but also relevance. 10k followers on a gaming channel won't help a literary fiction debut. They assess convertibility: how many of those followers are likely book buyers?
They'll also look at your professional online footprint: a clean, author-focused website, a LinkedIn profile that shows real-world expertise if it relates to your story, and maybe a history of published short stories or guest posts that show you can work with editors. It's about proving you're serious and not a one-and-done writer. A small but targeted platform, like a newsletter for historical fiction buffs, can be more valuable than a large, generic one.