Why using a puncture repair kit instead of a new inner tube? I am a casual rider (~30 km commute as often as possible) in an urban setting.

Reading this SE site to learn about biking (the resource is extraordinary, even though I don't understand every technicality) I see that one of the suggestions given on a regular basis is to have a puncture repair kit.

I ride with a spare inner tube and used a kit maybe 30 years ago - I thought it was an inferior approach to just swapping the inner tube (because you need to find the hole, prepare the kit, glue etc.). I see that I was apparently wrong.

What are the reasons to recommend a kit rather than a spare inner tube?

Is it because


of the cost? An inner tube for my casual riding is a few euros, I needed maybe one or two last year
of the reusability? One kit probably serves several punctures but then I would consider having both - in the unlikely case I get a puncture (statistically from past years) I would use the spare, and have the kit in case of a second one.
of the ecological considerations?
of the weight? This is a good point but I do not feel the extra tube in my backpack over such a short distance
You have to remove the wheel to replace the tube. A repair can be done in the frame.

On older bikes without quick release, and with current gear hubs, electric hubs, Nuvinci hubs, belts etc, you need a spanner and oftentimes, the gear adjustment goes back different and needs fiddling with. 

This reduces the advantage of tube changes. A fix is only 3 mins.

Hints: 


Once opened, small tubes of glue dry out very quick, so I always have an unopened tube in my kit. 
I also find that using the aluminium foil from the patch, over the tube opening, and screwed under the cap, keeps the glue alive longer. 
real sandpaper beats the hell out of the useless punched metal scraper thingy
you should have a square of canvas or equivalent for covering splits in the tyre sidewall