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Traditional outfit of any nation has three purposes:

a practical one.
This is clear: a clothes covers a human body and protects it from cold and humidity.
a social one.
Traditional costume allows to check "friend or foe" at a glance, without any communication, because members of any particular social group wear similar (or special) outfit. Example: Muslim women's headscarves, or rockers' leather jackets. If a person was identified as "friend", the next level of "control" starts: a costume shows what age and social group this person belongs to, is he/she married or not, how wealthy he/she is, where does he/she live (in what village precisely), how many children does he/she have, etc.
a ritual one.
Our "civilized" world almost forgot about this function of clothes. However, some details persist in a culture without any logic reasons (just "it is good to wear this"). Example: even nowadays knitted sweaters have decorative stripes around wrists/neck/bottom. Why? Because traditional shirts had red ("fire", protective) trim at the same places (this meant that a divine Fire protected head, heart, hands, and the whole body of a wearer). A clothes was thought as a border between one's body and an outside world. It protected a wearer from evil spirits. Also, specific signs (we can say, "decorative details") made a signal for spirits of ancestors: "Take care of him/her, he/she is your descendant".

In Russian traditional outfit, all three functions were blended. Example: a toddler's/kid's outfit consisted of the one and only article: pure white shirt (no designs, no embroidery), that was altered of parent's clothes.
A practical reasoning for this is obvious: a child grows fast (on the one hand), and (on the other hand) he/she wears his/her clothes carelessly - so there is no point to make child's costume of any expensive cloth, or to decorate it with hard-to-make (handmade!) embroidery.
But, there are ritual explanations. 1) In accordance with Christian (and pagan) beliefs, a child is innocent, and Heaven's Light shines above he/she. The costume visualizes this Light, so it must be pure white, with no any marks of an earth's life. 2) Spirits of ancestors remember a child yet, and protect him/her without any reminders. 3) Parent's clothes gives an additional protection to a child (similar to how a womb protects a fetus).
A social meaning of such a costume is: this human being cannot even dress him/herself properly, so he/she cannot be responsible for his/her behavior.

Sources (in Russian)
  1. B.Socolov, Y.Socolov. "Tradition Life of Belozierie Region" (issued originally in 1915, re-printed in 1997)
  2. E.F.Fursova, "traditional Costume of Upper Ob' Region"
  3. N.Sosnina, I.Shangina "Traditional Russian Costume" (encyclopedia and photoalbum)
  4. "Russian Traditional Life" (encyclopedia)