Micronations are, for the most part a modern phenomenon, and are to be distinguished from microstates (such as Andorra and the Vatican) and historical legal anomalies, (such as Gex and Moresnet), which have (or had) some recognition in international law. They have been created for a variety of reasons from interest and amusement to frauduent, and few have any recognition by formal bodies, whether national, international or international organizations. A few have some notability (including Sealand and Hay-on-Wye) but most are of little concern to non-participants.
In the Alternative Histories they increase in number and gain more formal recognition. This leads to changes in international law - most of the micronations are incapable of fulfilling at least some of the requirements of formal statehood, although most are willing to sign up to international agreements (non-proliferation, the peaceful investigation of outer space etc). International organisations also have to transform themselves to accommodate large numbers of small states, many of which are of limited duration.
For further information on the microstates in OTL see the Wikipedia page on the subject and the micronation wiki.