making life easier for printers
When printing two colours we obviously have to separate to two different plates. They will then be printed through two screens so that they line up exactly. If you have perfect registration, on your film production and on your press, that isn't a problem, the colours will print as you see them. It's normal for lithographers to print 'butt-fit'. Screenprinting is not that accurate though, there's always play in the system, the mesh straetches...
You will usually find that the print shifts, the paper shrinks, the mesh distorts... It will result in white lines between your colours, and white is very obvious.
The solution is to give the colours a slight overlap, just a point or a few pixels, enough to hide that glaring white line. The overlap might be slightly darker than either of the colours but will be significantly less obvious that the white line. This will, of course, make the blue object bigger, though not by any amount that would affect the appearance of the design.
Trapping can be added to colours in many different ways, here's Illustrator - give the object a stroke and choose the overprint fill in Attributes. The blue and red object will butt-fit and the overprinted outline will sit over the join.
You can also do the same in Photoshop, select your colour, modify (expand) the selection and save the expanded
area as a new channel.