Canva has a lot of premium features. The free version is good enough for most people but Canva makes their money on the companies paying for enterprise licenses. Canva is now looking to sell Affinity to those same enterprise users as well. Adobe gained massive market share due to how easy it was to pirate their suite. Canva is looking to try things a different way. Offering it for free will gain a lot more users than only those willing to pirate it. Once Affinity is common on every creative's resume, it becomes that much easier of a sell to enterprise that they need Affinity in their shop as well.

Simply put, they want to be Adobe but want a cleaner boost to their userbase than the piracy Adobe products were known for.