Maybe just because somebody asked for it. Often I've had
more than one option in front of me, and just went with
whatever the customer said they preferred. Which may have
a rational basis, or not. The next guy might want something
different, but if they don't want to pay for test fixturing,
qualifying the second package and so on, they can just learn
to like what's on the shelf at that point.
There's often a little sliver of time where "why?" matters.
Then it's a done deal and you get on with life.
Within the same edge-extent, the castellated pin does give
you about 55% more wetted area. The fillet is also inspectable
because it is aside, not under, the package body (this is a
big deal in some end markets). But there seems to be no great
love for the style, generally; you lose any lead compliance
that would pick you up some ability to tolerate package
CB
TCE mismatch and so on, placing the solder joint under more
strain and rate / risk of failure even if it's prettier and more
inspectable to begin with.
www.rfm.com/products/apnotes/pcb_web.pdf