Back in the Age of Sail warship resistance to shot was largely determined by the quality and thickness of timber used. Ships of the line were able to withstand some light and medium size cannonballs at a couple hundred meters range. This changed when practical high explosive shot was introduced; it was capable of massacring wooden ships.
Shortly thereafter iron and steel became affordable in greater quantities due to industrialised production and ships with metal armour plating (and still wooden support) became the new capital ships. The hevy weight of the necessary armour was a problem, and this problem only grew worse when by the late 19th century the big guns (about 234 to 304 mm calibre) became more relevant and with new propellants (mostly cordite) also much more deadly.
One solution to the weight problem was to harden the vital parts of the ship (bridge, boiler and engine rooms, main and secondary artillery and its ammunition supply, fire control observer stations) and leave most else largely unprotected. This was known as "Citadel armour".
The most extreme version of this was the "all or nothing" armoured ship, which had only thin (IIRC up to 25.4 mm ~ 1") fragmentts-stopping armour outside of the citadel.
|armour scheme of the U.S.S. Iowa|
The consequence was that damage to the unprotected areas - including collision damage - could put a ship out of service for months.
|HMS King Geargo V with a damaged bow after a collision in 1942|
This naval example was mentioned first in this text not because it was the first or most important one; it merely offers a nice term for the phenomenon of having varying degrees of protection, even including no protection in some portions: Citadel armour.
A much older example is a hoplite, who (in the most expensive version) included an all (bronze plate) or nothing armour.
Modern infantrymen tend to have four protection levels; bullet proof plate with kevlar vest backing, kevlar vest and helmet at most capable of stopping handgun ball projectiles, polycarbonate goggles capable of stopping weak fragments and some buckshot and finally nothing stronger than cloth or boot leather. This is de facto an all-or-nothing protection against rifle bullets at normal combat distances.
Modern tanks have the same staggering of protection. Frontal protection is typically designed to stop hits by the biggest tank guns, while sides tend to stop at most autocannon and moderately-sized shaped charge warhead shots. Plenty of its components are rather exposed, though. There's no more armour protecting the suspension as with Matilda II tanks, unprotected fume extractors were revealed to be a problem in 2003 and the external weapon stations and sensors are at most protected against rifle bullets if at all.
|Matilda II tank, with expensive cast steel side armour protecting the suspension|
The widespread application of the concept of citadel armour creates a challenge, as claims such as "protected against ..." are usually misleading; the protection is always partial only.
This creates a great need for spare parts (such as optics for tanks, tires and windows for armoured trucks). It also causes a drop in readiness during prolonged campaigning. An infantryman may survive a bullet to the chest nowadays, but the fragments of a grenade in his limbs can still put him into hospital and recovery for months. Tanks are notorious for having a high degree of attrition and piling up at repair workshops during mobile warfare, though a large share of mission killed tanks is repairable usually.
There's also the problem of incentives; a requirement to withstand some extreme threat on some surfaces may easily lead to a neglect of relatively modest protection for other surfaces (see the fume extractor problem). Armoured vehicles tend to have no protected stowage for the personal items of the crew, so a great many tankers have already been unable to change into fresh clothes or boil some water for coffee because their backpacks were perforated and burned beyond recognition.
The drawbacks of the citadel armour concept are bound to re-appear during every warfare just about every time, and they are equally bound to be underestimated in peacetime when they're not as obvious.