For me, having a goal of 2000 words per chapter gives enough guidance to be able to write a decent amount about not a lot (focusing more on description rather than content). If I didn't have this guideline, I think my chapters would be too uneven and the pace of the story stunted.
Obviously, it all depends on your own writing style, what you're writing about and how much writing experience you have. I just try to stick to 2000 because I feel those length chapters would print well in a book, which would be a long term goal once I've finished the story.
Although, if you've set a specific word count per chapter and you haven't yet reached it, but the chapter is building up to a natural close, there may be no point in prolonging just to achieve a certain number. That way, the chapter may drag on and so your writing could become less exciting.
For me it's to maintain consistency between chapters. I chose 2,000 because I'd be aiming for 50,000 words in the overall book and over 25 chapters. The specific word count also act as a plan for me. I know the plot of my story (obviously), but by applying a word count I know not to reveal too much in one chapter and get carried away- again trying to focus more on the description in my writing than the content and action.
However, I suppose it might not really matter how chapter lengths vary and the contrast of long and short chapters can have an effect of their own. I've noticed this is used a lot in crime novels, where there's one perspective of the detectives and another of the person they might be trying to find (their chapters shorter).
Do you plan a word count? And is so, what's yours?