Why sometimes some PCB designer add extra traces to connect two pin to each other?
That is a terrible board design. The person who made it clearly does not do it for a living and they opted to just use the autorouter. This is a huge mistake as 90% of the time, if you don't take hours upon hours to set it up properly, it does nothing but make a mess. In some designs routing like this could even lead to electrical problems.

Any real, experienced PCB designer would not route it that way.

EDIT:
Further examination has brought even more issues into light, as Marcus has pointed out. There are components whose leads are shorted by traces, parts with their leads shorted to the same ground plane, etc. If the board shown above was actually manufactured, it would not work. It is not a legitimate design.