I already went over striker plate adjustment on another page, but we didn't get into why they need to be there in the first place. It's very simple and you might already know, but I'm gonna go a little deeper.

If I had a brand new door in a brand new house, everything is pristine but the builder forgot to put in a strike plates on the door frame. This makes it quite easy for the door to be kicked in, because if you think about it, what needs to or is going to actually break to let the door open? I'll tell you it's definitely not the lock. It's not even the door. It's the door frame itself. Door frames are generally made from fairly soft wood, and small ones at that. The frame itself is attached to the 2×4s that make up the wall. So if the only contact the deadbolt latch is making is with a very small piece of wood that will break only in that one little spot where it needs to let the bolt pass through. Here's another problem, most people don't have brand new houses with pristine door frames. They have old used and abused door frames that have a ton of holes where strike plates have been repositioned over and over again. On some of these old houses here in Taylor have 100 year old door frames, and still no strike plate because there's nothing left to attach it to, leaving the door so vulnerable that it could break through just by falling into it or just leaning on it too hard!
Strike plates with the proper length screws are absolutely necessary for your door to be truly locked.