Adobe Illustrator is full featured, one stop solution Vector Illustration program from Adobe of which birth is date backs to 1984.
Adobe has various advance & vary useful feature one of which is (Smart) Effects for vector objects which are highly editable even after you applied to different objects.
As stated in Adobe Help Documents Adobe Illustrator Effects are described as follows:
Illustrator includes a variety of effects, which you can apply to an object, group, or layer to change its characteristics.
Illustrator CS3 and earlier included effects and filters, but now Illustrator includes only effects (with the exception of SVG Filters). The primary difference between a filter and an effect is that a filter permanently modifies an object or layer, but an effect and its properties can be changed or removed at any time.
Once you apply an effect to an object, the effect appears in the Appearance panel. From the Appearance panel, you can edit the effect, move it, duplicate it, delete it, or save it as part of a graphic style. When you use an effect, you must expand the object before you can access the new points.
One of that effect is round corner by which you can apply round corners to your current drawn rectangle & square shapes in your document which allows you to edit it’s value at any time.