Once you get going you'll use a real CRM like PipeDrive, SalesForce, data lockers, and who knows what else.
Just like with sales leads - while you're warming up you'll still want to organize things. Many / most startups don't have a product to actually sell. At the same time it's important to validate your ideas with potential clients. Our best (pre CRM) success has involved a Google Sheet for leads and then using individual Tasks for companies that engage in more serious dialog.