See first Bet Yosef and Bach in the cited siman. They are based on Gemara Arve Pesachim 105-106.
If I understand your question as "I understand that for a meal we need kiddush, but why is it forbidden to eat or drink a small quantity?"
The answer is in Bet Yosef in name of the Gemara folio 106b. This question is discussed and we follow the stringent opinion. Shabbat gives an importance of seuda to each eating, this property of Shabbat applies also to the duty to make maasser for a very little amount of fruits on Shabbat before eating them (see mishna Teruma 8, 3) .
It seems that the Rambam allows to drink water before kiddush based on the Gemara 105a, but perhaps, according to the Bet Yosef, Rambam (and the Gemara which is his source) did not allow before kiddush but before havdala. The Bach is convinced that the last is the pshat in Rambam.