Step-up rings let you use one ND filter on multiple lenses with different thread sizes. So instead of buying a separate ND for every lens, you just buy one ND (usually the biggest size you own) and use step-up rings to adapt your smaller lenses to that filter. It keeps your setup light and saves a lot of money.
There’s also a new variable step-up ring from H&Y. It’s basically a mechanical, adjustable step-up ring system that replaces carrying multiple individual rings. You twist it to match whatever lens size you’re using and it locks in. Super compact, super easy.
Whether you choose a simple set of step-up rings or go for the H&Y variable system, both options are effective and help you avoid buying multiple ND filters for different lenses.