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> So what creates that imbalance? 

Chess is a game of balance, where creating a local majority of force is
usually the key to achieving strategic objectives.So you are constantly
manoeuvring to concentrate your forces against a weak spot of the opponent,
or to strengthen your weak spots that are in danger of collapsing under the
pressure he musters.

So typically any constellation of opponent pieces can be cracked if you are
given the time to organize your pieces in a constellation needed to crack
it. So a game of chess is a constant race between concentrating your attack
force, and the opponent strengthening the spot against which you direct
that attack. Being allowed to do two quiet moves in a row (which is what
happens when the opponent loses 1 tempo) makes it more likely you will win
that race.

Only in positions where nothing can be achieved no matter what (i.e. a
static defense exists that has no weak spots weak enough to succumb to even
total concentrated attack of all enemy material), a tempo loses its value.
Such fortresses are quite rare. An example would be the 'Charge of the
light brigade' position (3 Queens vs 7 Knights, in the presence of King
and Pawns). Once the Knights sufficiently protect each other, the Queens
are totally powerless to inflict any damage to Knight army, and basically
have to stand by idly, watching the Knights gang up slowly but surely on
their King.