Birds need four basics: Food, Water, Protection, & A place to raise a family. A fifth important ingredient is Playtime! For this a sandbox is perfect.
Now, of course we don’t know if the birds are actually playing in the sand (probably not) or using the sand for cleaning feathers and eliminating lice and other critters. But it looks like play, and it is certainly fun to watch!
It’s a simple and inexpensive thing to provide. Buy a few bags of sand at your hardware store. Mark off an area with large garden stones. Try to eliminate all vegitation with weed killer first. Perhaps cover the ground with plastic. Then dump on the sand! Add a few shells or pieces of driftwood for an aesthetic outdoor garden decor touch.

You could also just fill a birdbath or any container of your choice with sand for a unique garden decor point of interest.
If your bird ”sandbox” is where your birdbath isn’t (ie: sand in front yard, wet birdbath in back yard) then you may have a different array of birdwatching in each area. Placing a “sandbox” near large bushes makes it convenient for the birds that nest in those bushes. Some birds might just prefer sand to water for personal hygine. (My research is still pending in that area.)
Anyway, happy birdwatching all you bird lovers!