In some cases, you may not use the Lead standard conversion flow to create opportunities in Salesforce.
You might end up in a situation where a lead is coming from one of your marketing channels (SEO, Social Ads, SEA,...) and be missing the revenue input from an opportunity which should be related to this investment.
Imagine that John Doe fills a form from one of your Google Ads campaign.
Your sales rep does not look at the lead and create an account, a contact and an opportunity.