I'll take your bait: Because for ages Mail.app had serious issues interacting with both Outlook-based and Google-based email accounts, including lost emails, inability to fetch email on a timely basis, and lost credentials. Those issues are lagely fixed (although not entirely), but I long ago left Mail.app behind and see little reason to come back to it. Also, I dislike Mail.app's user interface and it lacks several organizational features (particularly on MacOS) I've come to depend on in other email clients, including pinning and prioritizing email.
Spark is one of the few clients that was (and continues to be, if you read the article) free across iOS, iPadOS, and MacOS. That it now offers a Windows client is even more gravy for someone like me that likes its workflow and appreciates a unified interface across mobile and desktop devices.
Another thing to note is that Apple has never opened its private APIs for push email, and the iOS/iPadOS clients don't support IDLE, which means if Push email (and its battery savings) are important to you, you either need to use iCloud-based accounts exclusively, or use a client that has its own push implementation, like Spark.
Finally: I use email primarily for business. I have a personal iCloud-based account, and then several Google Workspace-based accounts that are high volume. Spark allows me to receive and organize that email in a way that is intelligent to me, rather than fighting Mail.app's conventions that feel very under-developed and basic.
Whether or not I will subscribe or continue to use the free version I have no idea. Need to explore the new version better and see whether the subscription-only features are important to me or not. But this knee-jerk against subscription software is tilting at windmills. Apple itself is actively lobbying developers to adopt the model. You're going to see Apple start to gatekeep more and more of their own software and services behind paywalls as well.
That's why Spark works for me. It may not work for you. Isn't it nice we both have a choice that works for us?