Note Amperehours (Ah) is a unit of capacity and not current ; when multiplied with Volts the result is Watthours which is a unit of energy and not power.
12V lead acid battery is built with 6x 2V cells in series.
You'd use individual cells if...
you need a different voltage than 12V, that requires a number of cells in series that is not a multiple of 6
long term maintenance is more important than low number of parts, for example you can use extra hardware to balance the cells to make sure the cell with lowest capacity does not overcharge/overdischarge and die, you can also monitor each cell and replace the ones that go bad instead of replacing a complete battery, etc.
reliability: say you have 12 cells.
You can make 2x 12V batteries of 6 cells each, and wire the batteries in parallel.
You can also wire the cells in parallel (making 2V cells with twice the capacity) then wire these combined cells in series.
In both cases you get 12V and capacity is twice the Ah rating of the cells.
In the first case, the weakest cell of each battery will die first. When one cell dies in a battery, the whole battery becomes unusable, so this means when the two weakest cells die, both batteries are unusable, and the whole system dies.
In the second case, the weakest cells will also die first, but they will probably not be wired in parallel. So, two of your parallel 2V cell bunches will only run on one cell, which means the capacity of the whole system will be halved, but it will still be usable until you replace the dead cells.