Fanfiction: Is it "worth it"?

This a question I've been interested in for a while now. As a disclaimer, I don't think any less of fanfiction writers, my question is closer to this: I have ideas for fanfiction. But everytime I want to write fanfiction, I feel like my time could be "better spent" creatively on my original writing. Like I'm doing a disservice to my own creativity if I build my ideas off a preexisting universe.

So: do you write fanfiction? Do you feel like it's the same as "original writing"? How do you separate the two in your own mind in terms of their merit, if at all?

Let me know your thoughts. Hopefully I haven't offended anyone with my question. :(

Edit:

Just wanted to say thanks again for all the awesome and thoughtful answers I got yesterday for this question. It made me super happy and I got to think about a lot. This community blows other ones out of the water.

I ended up starting out the planning for my fanfic as a result! Maybe I'll post it on Imzy one of these days. :3

Thanks again!