Is it a common feature for a writing system to include a capitalized variant of itself?
What is the purpose of capitalization in itself? Is it ever truly necessary for comprehension?
Linguistics Stack Exchange is a question and answer site for professional linguists and others with an interest in linguistic research and theory. It only takes a minute to sign up.Sign up to join this community
Case is by no means universal; it is found in scripts descended from Greek (but not most other branches of the phylogenetical tree which is rooted in Egyptian hieroglyphs; e.g. Arabic and Hebrew, as well as the Indic family, do not have case) and a few other scripts.
https://en.wikipedia.org/wiki/Letter_case#Bicameral_script mentions Armenian, Coptic, and of course Cyrillic and Latin, and a few others (though apparently Cherokee should not be there!)
Mayan, cuneiform, and East Asian scripts do not have case, so it's really not something you would postulate as a typical or expected feature by any means. We can thus emphatically conclude that this feature is by no means necessary; though it is indeed thought to improve legibility in the systems which have this distinction.
ALL UPPERCASE IS ANNOYING BUT PERFECTLY READABLE
The convention to use capitals at the beginning of a sentence seems to have developed from the practice of writing a decorative initial letter at the beginning of a script in medieval European writing, and have then been loaned back to Greek, but I could not quickly find a source with better details.
A supplement to tripleee's answer: Although most Brahmi-descendant scripts in India and South-East Asia do not have separate upper-case letters, Javanese does have 7 letters that are used instead of ordinary letters in all positions (initial, medial, final) in certain proper names. They are called “aksara murda” or “huruf kapital”. You can find them here: https://id.wikipedia.org/wiki/Aksara_Murda (in Indonesian only).
Just picking up on part of the original question - the purpose of capitalization. It is not "necessary" for comprehension, but it does make comprehension a lot quicker. Reading texts and inscriptions which lack capitals and spaces between words is a very slow process.
Mediaeval scribes used capitals to mark the start of sentences and chapters partly because their religious works were read frequently. In addition to the parts used in the Mass, there was the widespread custom that one monk or nun would read a devotional work aloud to their fellows during meals. They also liked capitals for important words such as saints' names etc.
Capitalization really caught on with the arrival of printing. Printed books were meant to be sold, so they were constantly re-designed to make them more reader-friendly. Hence spaces between words, punctuation marks, and the standardisation of letter forms.