Communities using Juicebox can leverage their reserved rate decisively when they want to make it more difficult for new members to join. Funds can still be received, but more of the newly minted tokens will be owned by the project itself. The current project members can use this to decide how they will manage their subsequent growth on a per-funding cycle basis.
When the project wishes to make membership more accessible again, members can do so by lowering the reserved rate.
There's currently a discussion happening in JuiceboxDAO deliberating if it might be wise to move its reserved rate from 35% to 50%.
The reserved rate can also be useful for other purposes, this is just one possible metaphor that can be used to guide decision making.