To elaborate on the existence of ů (u with ring above): The rule of thumb is that it appears if and only if you hear long 'u' inside native Czech words, which is true, but it's more or less coincidential. (Note that this doesn't apply to loan words: for example "kůra" means "tree bark", whereas "kúra" means "treatment procedure". They are both pronounced the same, kinda like 'koo-ra' with a rolling 'r'.)

The point is that in old Slavic languages, there was a long 'o' (ó) in place of the current ů. This has since differentiated in a number of ways in each of the languages - in Czech and Slovak it turned into "uo". While Slovak has remained in this stage (the diphthong is now spelled 'ô'), Czech (especially Bohemian dialects) went on to long 'u'. I've heard somewhere that the ring above ů is to resemble the original 'ó' sound, and for all I know, it may be true.

Anyway, the existence of ů has ethymological reasons.

And by the way, Balto-Slavic family of languages? Sub-family, perhaps. "Family" is a higher rank, and Slavic languages belong to the Indo-European family of languages.

Sorry if I ramble too much, I just like to discuss stuff like this. :)