To 'spare' often means setting something aside now so it's available later.

Future Use

When you spare something, it's usually not essential at the moment.

Not Essential

Using 'to spare' in this way often implies compassion or mercy.

It can imply avoiding unnecessary damage or pain.

Shows you're willing to give something you have to someone else.

Usually pertains to a small or reasonable amount of what's requested.

Implies something is kept in case it is needed, as a backup.

The 'spare' is ready to replace something else if necessary.