This unusually complex way of adding a suffix can be explained by its reconstructed origin: a voiced palatal fricative.
[1] In some instances -seen is added, e.g. huone (room) and Lontoo (London) thus huoneeseen and Lontooseen respectively.
Since the beginning of the 20th century it isn't included in the lists of standard Lithuanian cases in most grammar books and textbooks, and the prepositional construction į+accusative is more frequently used today to denote direction.
Later, it has often appeared in the written texts of the authors who grew up in Dzūkija or Eastern Aukštaitija, such as Vincas Krėvė-Mickevičius.
Certain fixed phrases in the standard language are illatives, such as patraukti atsakomybėn ("to arraign"), dešinėn!