i have seen many people spill out a little bit of liquid before they drink from the cup, what is the purpose of that?

Shalom, Thank you for your interesting question. I must admit that though I was aware of this practice from my learning, I have not seen people actually doing so – and the reason for this I will get to latter. The source for pouring off a little of the water from the cup before drinking it comes from the Tamud (Hullin 105b – 106a) which states "Abaye also said: At first I thought the reason why one pours off [a little water] from the mouth of the jug [before drinking therefrom] was the fear of scraps [that may be on the surface], but now my Master has told me: It is because of evil waters. [I.e., water from which demons had drunk.] A demon in the service of R. Papa once went to fetch water from the river but was away a long time. When he returned he was asked. ‘Why were you so long?’ He replied. ‘[I waited] until the evil waters had all gone’. In the meantime he saw them pouring off [a little water] from the mouth of the jug; he exclaimed. ‘Had I known that you were in the habit of doing this I would not have been away so long’. " This practice though not ruled as a halachic obligation, is quoted by the codifiers in reference to the laws of blessings. That is, if one is going to pour out some of the water, then one should do so before the blessings, and not between the blessing and the drinking, as this would be an interruption (see Mishna Brurah 206, 12). As I mentioned, I have not seen this done as a widespread practice. The reason is that there are many sources that indicated that the practices once used to protect us from the negative spiritual forces are much less needed in our times. Various explanations are given for this – but suffice to say that much of the mainstream religious world no longer worries about "water from which demons had drunk", and they have great authorities to support them. Blessings