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.