A more detailed explanation of Activity behavior flags

Can you please explain in bit more detail:
Load Workflow Context - When enabled, this will load the workflow context into memory before executing this activity.
Save Workflow Context - When enabled, this will save the workflow context back into storage after executing this activity.
Save Workflow Instance - When enabled, this will save the workflow instance back into storage right after executing this activity.
Save Activity Output - When enabled, this will store this activity's output as part of the workflow instance. Enable this when you plan to reference this output from other activities

I'm especially interested in last one. When does it need to be checked? I ask because a simple tests seem to show that I can use output of this activity in other activities even when this is not checked (provided activity has a name) (maybe I'm missing some more complex case).

In addition can you explain relation of last item to #898, my "naive" assumption would be that if I keep this unchecked it won't get stored anywhere (would be "volatile").....