This is how Russian distinguishes between "in" and "into".

в+prep = in (location)
в+acc = into (direction, motion, destination)
на+prep = on
на+acc = onto

Unlike in English, this distinction is upheld at all times in Russian.

It also exists for pretty many propositions regarding location:

под+ instrumental = (located) under
под+ acc = (to) under


Similar to the preposition 'с', which can mean 'with' when affecting an instrumental noun, but means 'from' (in a locational sense, rather than recipient/receiver sense) when affecting a noun in the genitive.

Он поднялся с колен. - He rose from (his) knees.