Say your are building a simple fence. What is not so simple is that every pillar must be in level and vertical.
The normal way of doing this is to have a spirit level, laser line level, water level or even theodolite.

Why not have a (self levelling) camera or two. or three?

Since the camera knows its orientation and its lenses and in turn optical aberations it can say what is level and in line and not.
Point out the pillar you want vertical and let the software in the camera do some image recognition and tell you when it is. Add another camera looking at the same pillar, but from another angle and the pillar’s angle is shown to you in all directions.

Now each camera has 3, or more, of the pillars in view and can tell you both if the pillars are vertical and if they are aligned and if they are level.

Use the same camera and software to help you with attaching the pickets equidistance.

If you give the camera some clues about distance, for instance by attaching or showing a ruler at some clever positions, it can return measures directly without you having to take out the laser range finder, carpenter’s/folding ruler or tape measure.

If you give the camera(s) more information with calibration points, like balls mounted on a stick stuck steadily into the ground, you can move the cameras around and the system will still know where it is.

Focus and focus point is a problem as optical aberrations might change when refocusing.

Say you and your spouse is shopping food together. You are both running in the same store and looking for the same goods. You have to check in with each other often to sync what each have taken and what you should continue with.

Instead there could be an app, or simple web site, that does the syncing.
Let’s say you have a shopping list stored in your phone. You give it, or you are given, a unique id. Give this code to your spouse.
As soon as you check any item on the list a signal is sent to the other phone, directly or over internet, to check the item on other phones using the same code.
That way you can go shopping together without having to go back and forth to the shopping cart to discuss.

Extra bells and whistles could be the ability to send small messages like “double the milk” or “do we have eggs at home?”