Even with 1-move arty, it's nice to have 2-move tanks. Retreat back when damaged faster, get back into the fight faster, spot of pillaging on the way, swat the AI's units when they shuffle them around between cities, kill two things when the opportunity presents...
Yes, the ability to attack twice by default is very helpful. They probably don't belong in this list of "useless" things honestly...they're certainly far ahead of serfdom. But people generally do overbuild them; they're terrible defenders and need a lot of babying. They are used like cavalry but they don't stomp previous era units anything like cavalry do to medieval

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But with fewer combat factors.
Perhaps...but with air power you're looking to dominate with the air power, and you most certainly can. Which units attacks the enemies at 50% str barely matters, and if you have air power anyway you can shoot down any interceptors. Paras tend to be better than tanks in practice if you have this kind of approach...but honestly if I'm not nuking I'd rather just flatten absolutely everything with ease using CR II or CR III artillery + chain vassals.
Naval bombardment doesn't actually oblige you to make an amphibious assault. "But I want to take the city before the enemy brings up defenders". Well, maybe - on the other hand, if my main way of reducing defences is naval and the AI has a mix of coastal and non-coastal cities, I may well encourage them to slot more units into a coastal city where they can be easily killed.
Depends on your goal. If you are willing to cap said AI you probably want to just take the city and then either burn it or turn it + 1 more city into a colony (lol). That way, you get war success against the AI and the AI can't get any from retaking the city from you (its war success against your junk colony doesn't count, and the fact that you colony has 8 tiles of shared border with the AI makes it more likely to cap in the mechanics trololololol). You can use this to repeatedly farm war success and weaken the target until it caps, which usually isn't long.
Naval air power is limited to fighters, though you can have a ton. With jet fighters you can bombard/damage inland targets easily too, though on many maps fighters will suffice. Marines win over tanks in these cases regardless, because even though tanks can move inland faster, they're weaker on initial attack and are inferior defenders. A mix might be useful, but you wouldn't want to just spam tanks there either and in most cases pushing inland is costly and unnecessary; you force cap and load right back up for the next target.
If you really want to take and hold land on a new continent though, better to just establish a foothold and then either ship or airlift in there. I like to ship because it's less micro intensive (since cities can't auto-airlift using waypoints for some annoying reason). One fun trick is to take a city, then unload some junky leftover units like chariots to pillage flatlands + guerrilla II whatever to pillage hills. Get rid of roads/rails and land stacks can't counter-attack the next turn lol. In fact, they'll put their nice big stack of "collateral me" right next to the city and let you mop it.
Of course, you could always just take a hill city and put 20 machine guns or so there too

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