In a SAFE, once your equity allocation has been filled, you cannot accept any more investments (at least, not without starting another SAFE). With the Rolling SAFE, once your initial allocation is filled, investors can continue to inject capital, and those investors will set your company's valuation.

In short, the SAFE gives you control of your company's valuation but leads to more dilution. The Rolling SAFE limits dilution and automates company valuation based on demand.