What's the use of the teardrop shapes around some PCB pads?  



There are primarily two reasons to use teardrops:  


It avoids a pocket (where the trace meets the pad) that could collect acid from the PCB etching process which would later do bad things.
It reduces mechanical & thermal stress resulting in less hairline cracks in the trace.


That being said, in professionally made PCB's teardrops are rarely needed. It's almost more of an aesthetic thing than a solution to a real problem. I've done many boards with and without teardrops and I have yet to notice a difference. In my opinion, they are more trouble than they are worth.
