In simple cases, that logic simple sets a toggle or whatever. In slightly more advanced cases it might modify something such as appending to a list. But in some cases it could be performing more complex logic such as applying a filter to a list. Sometimes this logic is only done client side (if the filtering is view-only for example) and sometimes both client and server. Basically, if you remove all of the network requests, it would still look like it’s working, more or less.

Of course only logic that is needed for data to be correctly displayed in the UI is needed client side. Performing logic that is only used server side (even if it has later UI effects — only effects that the user expects immediately need local representation) is unnecessary client side.

Eg even in a game, you might not want to run, for example, bot AI speculatively on every client but rather just the movement prediction like for other players, while everything else may be needed for rendering.

It would apply the updates optimistically to the client side DB, and the much of code that sat between the database could be shared between client and server. Neat stuff, even if overall Meteor wasn't my favourite thing to work with.