The Torah commands us to abstain from doing melacha for a 24 hour period. Hence the Shabbos has a fixed time slot every week. However the Chachamim have instructed us to add extra time on to the Shabbos (on 'erev and motzaei Shabbos;' TOSFEES SHABBOS). The Jewish people demonstrate, that although the Torah has regulated the Shabbos, they cherish it and wish to extend it.

For observing the Torah ordained Shabbos, wish a wonderful reward would not be offered, but since we observe the Shabbos and remove the 'restrictive' boundaries of the Torah, we are rewarded with boundless reward (MIDDA KENEGED MIDDA).