The curved area you mentioned is the neck of the bottle. The neck is designed to be narrow for several reasons and some of those have a direct impact on the curve. One reason for the narrow neck is to make it possible to seal bottles air-tight with a cap or cork. Another reason for the neck is that it's easier to pour from narrower openings. With a wider bottle opening, it's more difficult to control pouring liquid levels so that they don't spill over. A third reason for the narrow neck is to make it easier to grip and hold
The neck is the narrowest part of any bottle, and it's where it curves inward to form a lip on which you can hold or pour liquid. Bottles with an especially short or wide neck can be difficult to hold because they're not as ergonomic.
The first reason is that the consumer wants the drink in their mouth, not on it. If you were to hold your hand flat over your mouth while drinking from a cup or glass, you would notice that there is a large amount of liquid coming out and going into your mouth. With bottles, however, it's much easier to control how much liquid goes into your mouth as you're drinking because there are less places for it to go (unless you tilt your head back). This helps prevent spills!
The second reason is that liquids should be contained within certain areas so they don't spill out everywhere when transportation occurs via land or sea vehicles. The neck width allows this containment since no one wants water splashing all over their clothes while traveling on an airplane flight!
This preserves the liquid inside, keeping oxygen and bacteria out.
The slope of the bottle also allows for a narrower opening, which makes it easier to control the flow of liquid. A narrow opening means that less liquid will spill out when you are pouring it, and that you can control how much comes out by controlling how quickly or slowly you pour. This is especially useful if you're trying to measure exactly how much comes out because it allows you to get more precise measurements.
The rounded shape of the upper part helps keep liquids from spilling over and making a mess on your countertop or table. If there was no curve at all in this area, then liquids would start pooling up along all sides of your glass container instead of staying within its confines!
The wider the opening, the less control you have over how much liquid comes out at a time. This is because a wider opening allows more liquid to flow out at a faster rate. It's easier for liquids to spill over into your hand or on the floor when pouring from narrow openings than from wide ones.
If you think about how you hold a typical water bottle with its wide mouth, you might realize that your hand is holding onto two different areas: the bottom of the plastic and a thin strip of metal in between. Holding onto this thin strip can be difficult because it’s not particularly easy to get a good grip on it unless you use two hands (and sometimes even then). The curved area at the top of an S-shaped sippy cup helps make things easier by offering another place for your thumb and forefinger to rest comfortably atop one another. This makes it much easier for you to grasp and lift up your infant’s drink container without spilling any liquid!
In addition to being aesthetically pleasing, bottles are designed to be functional as well. For example, the narrow neck makes it easier to pour liquid accurately and the curved area on the upper half makes it easier to hold the bottle.
In conclusion, we can see that the curved area of the bottle has several functions. One of those is aesthetic appeal, but another is to make it easier to handle and pour liquids from.