Semi-OT: Carriers? When are they good?

[Starhawk]

Alright as all many of you know I am writing a story: Here and I have currently designed 2 carriers as follows:

Class: Hallice
Mass: 1300 kt
Designation: Planetary Assault Carrier
Fighter Compliment: (Standard) 41 Bullshark Attack Bombers
Armor: 20 Ablative Armor
Shields: 7 mk X Phased Shield Generators
Defensive Weapons: 4 mk V Defensive cannons, 4 mk IV shrike anti-fighter missile systems.
Propulsion: 4 quantum engines, Solar Grav Sail
Sensors: Multiplex Tracker mk 5, standard sensor suite
Defensive Suite: ECM level 3 generator
Crew: 5,000
Pilots: 82


[b]Class: Nimitz
Mass: 2000 kt
Designation: Mothership
Fighter Compliment: 68 Viper Fighters, 40 Bullshark bombers
Armor: 30 ablative armor
Shields: 6 mk X shield generators
Defensive Weapons: 5 mk V anti-fighter missile systems
Propulsion: 1 Quantum engine, 1 solar grav sail
Sensors: MK V multiplex tracking system
Defensive Suite: 1 cloaking device, 1 level III ECM generator
Crew: 8,500
Pilots: 148

Okay now that you've seen both carriers and their standard fighter loadout, I ask you this, when are carriers better in place of capital ships for getting a job done? For example the first action one of my Hallice carriers took part in was destroying 3 enemy planets with it's bomber wing.
Now the Nimitz class is designed to bring more variety to it's firepower, 68 heavy fighters for ship killing ability and 40 bombers for planetary attack but it's also a lot more expensive.

What situations should I use these carriers in? as I intend to keep numerous carriers of both models active in my fleet.