A number of reasons. For one, you pay per execution with Lambda (and other similar services like Azure Functions and Cloudflare Workers)...and the costs can be dramatically lower. Here's a great article by haveibeenpwned author explaining how they support 141 million requests a month at 2.6 cents per day: https://www.troyhunt.com/serverless-to-the-max-doing-big-thi...
Besides cost, automatic scaling is something traditional PaaS does not offer.
Besides cost, automatic scaling is something traditional PaaS does not offer.