Because then you would use the preposition a. Você vai à cama. In this case, the sentence is referring to preparing to sleep. Going to bed.
In this case 'para' is a preposition like 'a'. ex; você vai para algum lugar = você vai a algum lugar.
That is an oversimplification. Ir + para is more of a long-term situation. Eu vou para a Inglaterra em Julho. I will be staying there for a while. Ir + a is a more short-term situation, however is used less often in informal speech. Eu vou ao Japão de férias. I will go to Japan on vacation. So, in these rules, to go to the bed, not to stay, would use ir + a, while ir + para was used here. Honestly, this is just one of those situations where you just have to learn it, because as with any and every language, some things do not make much sense on the surface.
How come it is 'ir para' here and not 'ir a', considering being in bed will be temporary (or we should hope so). Or is this only when you go to a location?
No.. let's take a look, "ir a" is to stay for short time in wherever you go, ex. Eu vou à lavanderia, I go to the laundry. But "ir para" is to stay for long time in wherever you go, ex. Eu vou para casa, I go home and I'll stay there for a good time. The same way you go to bed, ir para cama/ir dormir, stay at in average for 8 hours.
Brazilian simplify it in the speech: we used "para" instead of "a"
So they consider it a long time when you go for 8 hours? But you say eu vou ao trabalho, not eu vou para trabalho (or at least, so I have been told). I also always understood that when you go on a trip, you would say eu vou a Lisboa, while if you would go there to live there you would say eu vou para Lisboa.
Yep you have the ideia. But I think "Eu vou para o trabalho"is also fine. Specific time isn't the problem, the choice of the preposition is about a subjective ratio, e.g. take the day time. and look to how many time you use when taking you children to school, now compare to 8 hours... See why we use "ir para cama" for "sleep/go" to bed and "ir à/até cama" for "to go to the bed"?
Of course that something we (natives) need to hit the head to understand, but we use a trick: always "para" in everyday speech.
We don't usually use definite article after "pra". Then, it should be "Você vai pra cama".