What is the halacha regarding a ketuba - do a husband and wife always need to have it with them (for example if they go away do they need to take it with them) or do they only need to know where it is?

They have to keep it in a safe place and know it is there.

what is the reason for this halahca?

I'm not sure what your question is. Are you asking what the reason is for a Kesuvah?

no, why do you always have to know where it is?

To make sure you have it and it hasn't gotten lost.