We use number and letter displays everywhere to show information. There are many different ways to display information, and technologies are improving all the time.
Seven segment displays (like the one in the title) were invented in the early 20th Century, but did not enter widespread use until the 1970s. They can easily display the numbers 0-9, but they cannot display all the letters, as they have no diagonal lines and are a fixed width.
For this reason, seven segment displays are mostly only used when only numbers are needed, such as for calculators, digital clocks or showing currency exchange rates. Some words can be spelled out using seven segment displays. Luckily for DVD player manufacturers, many of the words needed on a DVD player (such as no dISC, PLAy and PAUSE) can be spelled out, so seven segment displays are used for this purpose too.