Thinking back over 40 years.
When I was younger the need for a new rod was dictated by the rapid advances in materials through the 70's and 80's.
Wanting to switch from cane hand me downs to an East Anglia glassfibre rod and then to carbon.
The rest was down to marketing and wanton gullibility.
Do I really need 8 (or more) different feeder rods ranging from 6' to 13' for fishing the same river 52 weeks a year. Probably not, but as long as I convince myself that they can be used for that one specific half hour on the third Thursday of December every four years when the wind drops and the moon is in the third quarter then I'm happy.
I think also that 'age' of tackle plays a part. Sometimes it is just nice to buy something new, for example my main feeder rod is a twenty year old shimano technium which is still a great rod. But over the last few months I have started looking for a replacement even though it is not needed.
But to answer Lord Paul's question. The last thing I bought which I truly needed was my 14' ultralight.
I had float rods of 10', 11', 12', 13', 15' and 17' (some lengths duplicated) so as you can see there was an obvious gap.