De Wikipedia :
The PC speaker is generally the first output device to be activated during the boot process of an IBM-PC derived computer. Since it is active before the graphics card, it can be used to communicate error codes related to problems that prevent the much more complex initialization of the graphics card to take place. For example the Video BIOS usually cannot activate a graphics card unless working RAM memory is present in the system, while beeping the speaker is doable with just ROM and the CPU registers. Usually, different error codes will be signaled by specific beeping patterns, such as e.g. "one beep; pause; three beeps; pause; repeat". These patterns are motherboard specific and are usually documented in the technical manual of the motherboard.
se você ler mais, você pode pegar onde eles são usados. e sua última pergunta será relacionada com o tipo de placa-mãe ... mas, com certeza, raramente é usada hoje em dia.