Originally Posted by herolee10
From what I've heard and read, I think that was the original intent of that symbol; so that villains would aim for that spot of his costume due it having a protection underneath, whereas the rest of his body didn't have as much.
Actually, I'm pretty sure that rationalization came later, from Miller's The Dark Knight Returns, if memory serves. It was an excuse for a silly silver-age costume choice to be used in the "dark and serious" story there. Again though, it raises the question of why Bruce would allow his underage partner (that time an inexperienced young girl) to wear such bright colors as Robin if his own bright colors were intended to draw gunfire to them.
The real reason for the yellow oval is just that they were looking to reinvigorate the Batman brand after a slump in the mid 60s or so. The result was the "New Look" Batman and the coming of the camp
. The design stuck around for a few decades before being dropped during the No Man's Land arc, even though Batman had since gone from camp joke to serious antihero. It's popped up a few times since, like the Earth One Elseworlds stuff or Batman Inc., I think, but it's no longer the norm, probably just because Batman is seen as 100% serious in most continuities now and the design doesn't mesh well with that.