The present day structure (Marian column (Prague)) is intended to be a very precise reconstruction of the original one built in 1650 that was torn down in 1918, when the original inscription was destroyed.
— The Maria column replica, Prague, Old Town Square, plinth detail.
The inscription was and is in the form of an eteostichon or Chronogram:
A chronogram is a sentence or inscription in which specific letters, interpreted as numerals (such as Roman numerals), stand for a particular date when rearranged. The word, meaning "time writing", derives from the Greek words chronos (χρόνος "time") and gramma (γράμμα, "letter").
In the pure chronogram, each word contains a numeral; the natural chronogram shows all numerals in the correct numerical order, e.g. AMORE MATVRITAS = MMVI = 2006. Chronograms in versification are referred to as chronosticha if they are written in hexameter and chronodisticha if they are written in distich.
The text reads as:
VIRGINI GENITRICI SINE ORIGINIS LABE CONCEPTÆ, PROPVGNATÆ AC LIBERATÆ VRBIS ERGO, CAESAR PIVS ET IVSTVS HANC STATVAM PONIT
~ The Emperor erected this statue of the Virgin Mother of God, conceived without the stain of original sin, out of pious and just thanks for the defense and liberation of the city.
And as Czech Wikipedia details, the numerals on this one are meant to encode the year 1650, referencing
— TEIGE, Josef ; HERAIN, Jan. Old Town Market in Prague . Prague: Society of Friends of Czech Antiquities, 1908. 430 pp. Available online . pp. 241–249.
Note that this is classified as:
The Marian Columns were built in Bohemia and Moravia often as commemorative columns, as a reminder of the tragic events, especially the wars and the plague epidemics.
The chronogram is located on the sandstone pedestal. The Roman numerals of chronogram are larger than the other letters and are red color-coded from the rest of the text.
The year 1650 from the chronogram referring to erection of original Baroq column.
Type of numerals used: Roman
Text of the original chronogram: VIrgInI genItrICI sIne orIgInIs Labe ConCeptæ propVgnatæ et LIberatæ VrbIs ergo Cæsar pIVs et IVstVs hanC statVaM ponIt
Pure chronogram?: no
Natural chronogram?: no
For a deeper 'reason(s) for doing so':
The embedding of coded dates (chronograms) in religious and secular memorial inscriptions became something of an obsession in the early 1700s, especially in the Czech lands.
— Alex Went: "Digital Signatures of the Baroque. The Cult of the Chronogram in 18th Century Bohemia", CRIS Bulletin, 01, 2016. doi
The Roman numeral code:
VI + I + I + I + ICI + I + III + L + CC + V + LI + V + I + IV + IV + V + C + V + M + I
might be rearranged in big endian MSB order as
M CCCCC LL VVVVVVV IIIIIIIIIIIIIII
which is in Arabic decimal addition
M = 1000 = 1000
CCCCC = 5 x 100 = 500
LL = 2 x 50 = 100
VVVVVVV = 7 x 5 = 35
IIIIIIIIIIIIIII = 15 x 1 = 15
So, for a much shorter and natural chronogram that would match our expected notation: ~Maria Defenditur, Cæsar Liberatur? But I guess that would be less baroque and more laconic/bauhaus.
Other examples for coding that specific date/number, with perhaps better Latin and even more fitting content for such a column might be:
Me Do CeLo. or: Magnas ferte Deo grates pro paCe reLata. or: CeDant arMa togæ toto toga fLoreat orbe. Which are all found in the following collection, which lists many decoded examples, although not the Prague one:
— James Hilton: "Chronograms, 5000 and more in number, excerpted out of various authors and collected at many places", E. Stock: London, 1882. archive.org