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 DarknessEternal wrote:
You don't.

Marines have several competitive options that include no vehicles.

QFT

Vehicles can:
Act as cover to your infantry,
Get up close early and effectively,
Easily survive small-arms fire,
Hide your infantry from small-arms fire and light AoE weapons.
Move your infantry around when speed is needed,
Move infantry across terrain easier than walking (through area terrain, or through rivers, etc),
Carry bigger weapons than infantry squads could,
Resist (lots of) psychic powers better than a squishy mind.

What they cannot usually do:
Score objectives,
Contest objectives,
Climb or enter buildings,
Survive a Melta shot (or two),
Assault the enemy.

So, there are pros and cons, and you can easily ignore them.
But a bunch of vehicles in your list changes it entirely.

You don't need vehicles in 6th edition so much as you need mobility. You either need to be able to move in close enough to strike or you need weapons that shoot further than your opponents. There are a lot of armies that have outstanding shooting options, so mobility is especially important for cc / assault type armies.

I take Rhinos for my CSM army specifically to get my guys to close enough to do damage. I play a lot of lists with Noise Marines and Flamer Chosen, and it's essential that they make it 12 - 24 inches out of my deployment zone for them to do some good. My Rhinos get shot up, it's a fact and it happens. The way I think about it, I am paying around around 140 points just for the benefit of getting up the board. I don't need those vehicles for anything else.

Those Rhinos are also magnetized. I sometimes swap out their doors to make them into laspreds with havoc launchers and bring them, along with helbrutes with twin-linked lascannons. the ability to drop a ton of lascannon shots into my opponents can also be valuable, and there's simply no way to do that other than vehicles. In these lists, I am paying for weapons that can one-shot enemy commanders and tanks, and can survive just a little longer than a Rhino.

But vehicles don't survive long in 6th edition, they need to play a role in a bigger plan and it needs to be clearly defined. In general, it's to move other units up the board quickly or to bring big guns. I don't see a use for them other than that.

the guy who i battle the most plays with GK and i really, really hate his purifier list, but it's a good example for your question.

His 5 model squad purifiers alone won't harm my nob bikerz alot and they would be shot to grottpieces by my lootas long before they reach them by foot.
but by putting the purifiers in the tracked turbocharged lunchboxes, they gain sort of an additional life..

you can shoot at a rhino and don't do enough damage, so he keeps on joyriding on the battlefield. If you manage to destroy the lunchbox, the infantry is out (but your shooting with that unit is over) and they hide behind the wreck for your other units that can still shoot. then its his turn and you see 4 more rhinos approaching to soak a single shooting attack or charge before you can reach the meal inside. And if you think of it, if you blow up a lunchbox, you probably have delivered alot of extra wounds/pen/glancing hits who just go lost in space if the box is broken.

that's the reason i would take vehicles.. you can throw nuts at your enemy, but a tracked nut in a shell is faster, and has the extra 'life shell'