Datasheets are written by engineers for engineers and can sometimes be hard to understand. In a datasheet, you will most likely want to look for a piece of specific information about that device. You can also use it to choose the correct device for your application.
For example, suppose you want to know which transistor will be best for your power supply design. In that case, you can use the datasheet to know its voltage limits, power handling, current carrying capacity, or the input impedance of an op-amp.
As you would expect, there are thousands, if not millions, of different types of components and their corresponding datasheets. Even the same device made by different manufacturers may have differing datasheets.