I like that you made a model specifically to learn how something as important as bonded contact works.
One way to get a good understanding of what bonded contact is doing is to set the Formulation to MPC.
When you do this, after you solve, click on the Solution Information folder and the main window switches from Graphics to Worksheet. Click the tab on the main window back to Graphics and you will see a plot of the Constraint Equations (CE) that make up an MPC Bonded Contact. You can see and example of them in the image below where the flat side of a sheet metal box is bonded onto the U shaped part and some internal stiffeners.
Another important feature is Pinball Radius. On your test model, make the large flat plate have smaller elements, instead of 5 element across the width, make it 25 elements. I don't know the width of that plate, but say it is 100 mm, so each element would be 4 mm wide. Now try setting the Pinball radius to 5 mm and solve. Then set it to 10 mm and solve. Then set it to 25 mm and solve. Look at how the red CE elements reach further out across the plate. Please try this out and reply with some images. Thanks.
Note that if you set the Pinball radius to <1 mm, the solution may fail because no nodes on the plate will be close enough to form a CE.
2) There will be some difference between sharing nodes along an intersection line and using bonded contact. Large increases in stiffness is not a big problem with Bonded Contact, it is a big problem when a Remote Displacement or Joint is used with the Behavior set to Rigid and the scope is a large face.
3) Bonded contact can be used in Large Deflection nonlinear analysis where large angles of rotation are present. Bonded contact should be avoided if material plasticity is present in the area of the bonded contact. The help explains that you can have a Linear Model, which solves fast, and also use Bonded Contact. That is not true if you use Frictional Contact.