Our Rabbis established Berachot (blessings) that need to be made before we eat and drink because a person cannot derive any enjoyment or benefit from this world without acknowledging Hashem. Therefore, when a person eats or drinks and it provides nourishment or good flavor or taste, the person has to make a Beracha (blessing). It doesn't matter how much or how little the person has: even a crumb of food or a sip of a drink that the person enjoys and gets flavor and taste from means they are going to have to make a Beracha.
This is different from Beracha Acharona (post-eating or drinking). When it comes to making a Beracha after we eat or drink, it depends on whether we had the proper shiur (proper measurement and amount), because the Torah says regarding Birkat Hamazon (Grace after Meals), ve'achalta ve'savata u'berachta, they have to be satiated before you make a Beracha. So when the Rabbis modeled Beracha Acharona, post-eating or drinking, they said there has to be a proper measurement and amount for a person. If they don't eat a kezayit (an ounce of food) or a revi'it (three ounces of liquid), the person is not going to make a Beracha afterwards. But on any small amount, they would make a Beracha before they make a Beracha. Additionally, when it comes to making Beracha Acharona, not only do you have to have the proper amount, but it also has to be within the proper time frame. The person has to eat a kezayit, an ounce of food, bichdei achilat pras, within the time frame of what the Rabbis call eating a half a loaf of halachic bread, which basically is somewhere around six or seven minutes. It's somewhere in that realm. So if a person does that, then he's going to be required to make a Beracha Acharona. But if a person is just picking a little bit at a time, even if after, let's say, a whole hour or two hours, they ate a kezayit, but they're eating very, very slowly, a little bit at a time, the person is not going to make a Beracha Acharona. The same goes for beverages. This is why generally a very hot beverage, like a hot coffee or a hot tea that a person is sipping is not going to be a Beracha Acharona.