As someone who wanted to tinker and personalize their devices, the creator of this app wanted a simple way to customize the "limit reached" screens AKA shields. The target audience could therefore be said to be people who LOVE to personalize and customize. Another target audience might be people who want a simple way to set limits on their app usage with no fuzz. Shields are easy to share with friends, so you don't have to create your own unless you want to.

OneSec is an excellent app in its niche, but our goals are different. While both this app and others set limits for app usage, their focus is different. Other apps focus on habit forming and various flavors of that. ShieldStudio has a different goal: Customization and personalization. Our main goal is to let users make the "limit reached" screens (aka Shields) as close to their vision as possible (and as personalized as Apple let's us). Before selecting this or any other app, decide what is important to you, and what you want to accomplish.

A Shield is the screen you see when you have reached an application or website (or group) usage limit. This wording is actually from Apple developer documentation.

No, there are no management of other devices built into the app. Feel free to use the contact form to express your interest if this is something you want.

The simple answer is that it was originally going to be a paid app. There are several reasons for it now being free: Most importantly new(ish) EU regulation that would have required the author to make a company and have a company address. That cost a lot of money, so it seemed wasteful for a niche app that wouldn't even make back that initial investment. The author got to play with Hugo and Cloudflare Workers, so it was not all bad! Learning new things is more important than money.

Yes, actually there is. She has a blog with an about page that covers that.