Why does this type of panel have two main breakers?

Now it makes sense.
Inside is so important,one of the many reasons I take close to 200 pics a job.
Nothing like good macro.

Here are a few more pics.

Here you can see the bottom half.

Why two different amperages tied together like this and is the solid ground wire proper? When the doorbell circuit is double tapped do you call it out or is it OK as in this arrangment?
Aulden