What and why we collect is always a personal thing. Condition of models and packaging is something else though.
Older toys (and some modern repro's) were sold in individual cardboard boxes, therefore the term "mint and boxed". Grading these models can however be a headache, both car and box should be "mint" (no visual wear whatsoever).
Should the box have "shelf wear", torn off side flaps, creases, price tags or ink markings, only the car could be graded as "mint". Many models came in mint boxes with some or other film. Errors on cars were very scarce due to the old school way of life where there was pride in everything, from the die caster, packer and quality inspector to the box.
Whatever the case may be, mostly everyone try to collect his/her toy cars in the "mint and boxed" condition. Already in the mid 60's did companies such as Matchbox (Lesney Products) and Corgi (Mettoy Playcraft Ltd) make blister packs for their models. Nice and easy to see in store what was available and which ones did we have to collect.
Problem is...once torn off the blister, it was only the car which could be well displayed. Models from those eras still intact on original blisters are therefore very much collectible, that includes old Hot Wheels.
Many collectors buy Limited Editions and take them off the pack - it is a personal thing - when displaying the scarce model next to a mainline, one find it hard to distinguish the gem from the others. Boxed models can be displayed and should, if the collector has ample space, as car and box. It has always made the cars complete, displaying together as a unit.
If we just want a specific model car, the condition should not really be a bother. Novices tend to fall into that trap. Whether we collect loose models in any state of wear or just models in ORIGINAL packaging, it's your personal thing.
From my side, collecting my stuff will always be with "mint and boxed" in mind.
PS. That does not include reproductions!