The intrepid class variable warp nacelles were starfleets response to the subspace damage caused by warp drives of conventional warp drive and nacelle designs like that of the Galaxy Class but not seen again on any other class of starship due to advances in warp drive/warp nacelles designs and efficiency

I think it was supposed to increase engine efficiency (and obviously just look cool and different on the show), but it was really silly, didn't make any sense (since no other ship needed engines like that), and never should've been incorporated in voyager's otherwise great design.

@Michael Benucci idk I disagree, it was a great touch to the intrepid design. As for why it was the only class ship with VGNs if I remember right from Memory Alpha it was explained as being un nessasary to have that system on other ships due to the changes made to other class starships warp drives and incorporating the data gained by the intrepid classes which is why we see ships going away from the large broad stubby nacelles like those of the Galaxy Class. Though don't quoat me on that as I may be incorrect however

Better power use

@Bluehemiram fair enough, that makes sense and I know it's just a matter of personal preference. Just to clarify, while I personally just didn't see the need to have a special nacelle design for voyager (especially since every other ship ever used by the federation was very effective without variable geometry pylons), the design was still fine and pretty cool overall. It just wasn't my favorite, since I don't think it added much to voyager's performance or style (despite them coming up with a technobabble way to explain it for the show).

@Michael Benucci oh for sure I do see what you mean as well. From a warp power standpoint I don't think there was much of an increase other then longer high warp travel time to use a car parlance to me it was like they gave the ship cylinder shut off which in turn made the ship more power efficient during ftl the need for variable geometry macelle pylons went the wayside after they used lessions learned through the use of the intrepids but I do agree with you they really could have done without it. I myself never particurly liked the look of the ship with the nacelles raised