The free plans include Preceden branding on some export formats (like PDFs) which serves as a marketing channel because anyone they share the PDF with will see

The free plans include Preceden branding on some export formats (like PDFs) which serves as a marketing channel because anyone they share the PDF with will see the timeline is created with Preceden and some will go on to check it out and become customers. Both paid plans remove the Preceden branding, which is one reason users might choose to upgrade. Regarding adding a custom logo, that's mostly a need of professionals in larger businesses (think project managers at Fortune 500 companies), so having that on the top paid plan encourages those type of users to upgrade to that plan vs the cheaper one.