This is what I think, as a recorder player and as a pianist. So I risk getting my head bitten off (always wary on the forums...). On recorder, you get the dynamic contrasts by using - well not tricks exactly, but you can't without (often unsatisfactory) 'leaking' of holes or alternatives get true dynamics. On recorder, if you want to emphasize a note, you'll shorten the note before, you might fractionally pause on the note or fractionally delay it. You use the most subtle of rubato to get the effects you want eg you might make the most marginal of accelerandos to give the effect of a crescendo. I emphasize these are all very subtle. In order to get variety you use different articulations and even more variety by using different consonants for the tonguings. So you use detached, or staccato or tongued legato or slurred etc.
I think harpsichord is akin to recorder and presents a similar range of solutions. You have to give the effects of contrasts and different nuances by other means than true dynamics.
On piano, I see it as doing all these things and you have the added bonus of being able to play dynamics as well. So that's amazing.
To answer, why would you detach quavers; I can only say that sometimes you do, sometimes you don't; that there is no 'right' way, that you play and listen to as much Bach as possible, that you decide which performances you like and why and gradually you develop your own thoughts on what you like to do. And it's not the same every time; I often play, say, the first two-part invention and it's different every time.
I second listening to Angela Hewitt; listen as she comments on the Gigue from the fifth French Suite.
As in all music, you gradually get a feeling for what is appropriate. So, for example, you'd know when a playing of a Chopin nocturne was either exquisite or way over the top. You know when a playing of Fur Elise is just the right speed - and when it's too slow, too sloppy and sentimental, when it's too fast, when it's too mechanical. And when it feels right for you.
I think, anyway.