I'm curious why PCBs have one square pad in row for 0.1" headers. What is purpose of putting one square pad? What it indicates? Can you push more amps through square pad than round?
This is to identify which pin is "pin 1".
In the case of a single-row header with no orientation key, this might not really matter, but the layout tool might do this by default and the designer didn't think to turn it off.
If this were to be used with a connector that does have an orientation key, I'd much rather also have a pin 1 indicator in silkscreen, so that the orientation can be checked after the connector is loaded (and without having to flip the board over).
Can you push more amps through square pad than round?
No, it won't cause any significant difference on the circuit performance.
(As pointed out in comments, for an RF or high-voltage application you might need to consider more carefully before using this method of orientation marking)
Not a standard, but it's useful to mark where pin 1 is. This might be useful if you don't have a silkscreen, but you wish to mark the orientation of the socket.
It is there to mark the location of a specific pin (mostly pin 1). It helps in deciding the orientation in which a component should be placed.
No you can't push more amps through the square pad. Check to see if the trace running to that pad is wider than the other traces, if not than it is similar to the others.
The square pad doesn't make a electronic difference when compared to round pads.
It indicates the location of pin 1