The problem starts when you fish for multiple species and sizes of fish through out the year.
After a while you start wanting dedicated setups rather than being under/over gunned with a "do everything" rod.
There are also times where a couple rods setup differently allow you to fish differently without wasting time switching things over.
Lets be honest, most times fishing there is more casting than catching going on. In that case its nice to have a rod that is light, has a nice action and feels good in your hand and is pleasing to look at.
Next thing you know, after a while, you have a collection of rods/reels/lines the newest addition usually nicer than the last, and you have accidently become a gear wh*re.
This process seems to accelerate if you are into fly fishing with the endless line selections, rod weights, rod actions, line tapers, sink rates, etc.