When beholding the tranquil beauty and brilliancy of the ocean’s skin, one forgets the tiger heart that pants beneath it; and would not willingly remember that this velvet paw but conceals a remorseless fang.
– Herman Melville, “Moby Dick”, Datalinks
The pressure dome is a very weird building. What it does is that it allows a base to be submerged under water. Since this is a very situational thing that only ever happens on rare occasions, there really is no need to build it unless you specifically find yourself at risk of being submerged. (It also acts as a recycling tank, but at twice the cost, it’s easier to just build the tanks.) From a gameplay perspective, the pressure dome is an edge case.
It is interesting to ponder the in-universe reasoning behind restructuring a base in such a way that it could withstand a sudden onslaught of ocean, however. Not merely a momentary high tide which sweeps across the base to then recede in short order, but a permanent submersion under the waves. This requires being able to resist an immense amount of pressure, both from the sheer weight of the water, and also from its moving around back and forth. Overall, it is a massive feat of engineering and architecture, which should be commended.
The ordeal is made easier by the fact that a base is a sealed system from the word go. Everything is already indoors, so there is no need to enclose the plazas, street grids and other public amenities; all that is needed is to harden the exterior shell of the base such that it can withstand the water. At some places, this means strengthening it structurally. At other places, this means smoothing out surfaces to minimize the areas that are in actual contact with the sea. Only the edges of the base have to withstand the pressure; the interior only has to withstand the whims of its occupants.
Given that this is an extensive effort in preparation for an unlikely event, I imagine that the cases where a pressure dome is actually built is marked by a high degree of anticipation. On the one hand, if it is deemed necessary then it means that the ocean is on approach, arriving sooner rather than later. On the other hand, there is no real way to know whether the dome will actually withstand the pressure until it is fully submerged. If it holds, it holds. If it does not, then the base is flooded with water and subjected to the remorseless fang Melville alluded to.