Imagine you have a the class NPC wizard in their tower or library or whatever. For them, its not a day of adventuring, not a day they expect to fight heinous evil....just good old research, some scrying, maybe a little divining, etc.
Now in said Wizard's perfect world, they would prepare no combat spells. After all, every spell prepared to blow something up is one less spell you have to do actual useful things. But....the world is a scary place, and you never want to be caught unprepared, so for your "personal security" you do maintain some combat focused spells.
Now the question....what would those spells be? What do you think a 9th level wizard who wants to be prepared for invaders (general trouble, nothing specific), but wants to utilize the minimum amount of prepared slots towards the effort as possible, would prepare?
Lets assume an 18 int wizard, so a total of 13 prepared spells in total. If it matters for your discussion, here are the spell slots per day such a wizard has access to:
5th - 1
4th - 3
3rd - 3
2rd - 3
1st - 4
Floating Slots (assuming one short rest) - 5
Also lets assume the wizard does NOT have access to magic items. So his own spells are what he's got.