One thing I notice about Trek starships is that they seem to become more 'streamlined' over the decades, from the original TOS enterprise through the refit, the D, the E, etc. I definitely prefer the more tall 'naval' silhouette of the earlier ships; they have more presence to me. But that's just a me thing. That said, I'm not clear why they would become more streamlined, as they operate in space. It's not like the evolution of cars and planes, which have to worry about air resistance. And the Borg certainly prove that streamlining in space is completely unneccesary. I guess it's just starship fashion trends?
The Kirk refit (ST1-3, and I guess the A in 5-6) is still the best looking Enterprise to me, though I love whatt hey've done with the earlier TOS Constitution class in Strange New Worlds. I'm such a starship nerd!
While space is pretty much vacuum, it still contains some stuff - radiation, random particles, the occassional micro-asteroid. And while it's very, very little, starships in Star Trek really fly fast, and all that stuff adds up. I might be just enough that if you fly at warp, it's cooler if you can reduce your cross-section against that a bit. If you consider that the deflector dish's job is to protect ships from that stuff, it makes sense to reduce the amount of surface it needs to protect.
Also, from a tactical perspective, if you chase someone, someone is chasing you, or you are intercepting them so they don't get from A to B, the front and the rear are probably the most and longest exposed to enemy weapon fire. If they come at you from the sides, you better turn towards them to fly past as quickly or possible, or turn away from them to fly away as quickly as possible. So it might be nice to lower your front and rear profiles as well.
There might also be reasons in the fictional mechanics of the warp drive that might make it desirable to be more "streamlined" - or maybe it's actually technological advancement that allows better streamlining (because you can make your warp bubble more enongated and still have it tightly around the ship, keeping it efficient) for the benefit of the aforementioned factors.
Of course, on the out-of-universe reason is - streamlining is an easy visual tell for people to identify something as "faster" or "more advanced".