As in the case of a brick facade, plaster can crack under pressure, as the foundation of the house settles unevenly. There are two choices here (until damage to the supporting structures has started): either permanently "grease and patch" the cracks, or solve the main problem and prevent future problems.
At first, you might think that it is simply the plaster material itself (although this may be in the era of "artisan" cheapening with non-compliance with any technical conditions and regulations), which shrinks and cracks, creating no more than a cosmetic problem. However, these cracks often indicate a serious problem lurking below.