Humble Advice From The Filter Drawer - The Step Ring Tango
Now if you're a filter user - and there are some who are, you know - the step ring is your friend. You might have a stable of lenses for your camera that have all sorts of different filter diameters - from 39mm up to 95 mm.
If the manufacturers of the lenses wanted to play the game fair they will have used the classical sizes - 48mm, 49mm, 52mm, 55mm, etc. If they were drinking sake at lunchtime they made them with 40mm or 56mm or somesuch nonsense size. There is a special curse for lens designers that specify non-standard filter sizes...
With the regular ones, you can select a filter that is the size of your largest diameter lens - say a 72mm diameter big zoom - and then get adapters to let you use this lens on the smaller lenses in the bag. If you only use UV filters it probably makes more sense to get a dedicated one for each lens and to keep it on all the time. But if you are a polariser user, you might spend your money wisely on a really top-grade Circ Pol and spread it over a number of lenses. The adapters are generally under $ 15 so it makes sense.
If you are dedicated to spoiling the resolution of your camera lens by clapping optical adapters on the front of the standard lens * the step-up and step-down rings let you do this as well. Video camera enthusiasts seem to want to do this a lot and perhaps it works for them - still camera shooters generally do better getting a fresh lens rather than trying to modify an old one.
Experimenters who pursue other optical effects - putting cameras on microscopes or telescopes - or who construct strange light boxes with matt board and gaffer tape** can also use the step ring as a ready attachment to the front of the lens.
* Exception to this rule - the converters that Fujifilm make for their X-100 series of cameras. Both tele and wide converters screw on to the front of the camera lens and do their job without degrading the camera lens performance one tiny little bit. Hooray.