In all my practicing I've never used headphones. Why do people use/need them?

Edit: Holy shit. This subreddit is terrible. So many people's responses make them sound like they read the title, misinterpreted it, and then responded to that alone hastily without reading the post. Thank you to the few people who chose to be helpful instead of being condescending, rude, or just downvoting everything I've said to hell. I had simple curiosities and most of what I got was toxicity.

I thought this was a subreddit where people could come when they wanted to learn about DJing. To get better, to hear from more experienced people, to get advice, to chat about ideas and topics. Clearly this is not the place, despite a few people actually trying to be helpful. I'd never want most of you teaching me even the simplest thing about DJing. Done replying to comments. Thanks again to the few helpful people. Peace.

Original post:

I've been practicing DJing for about a half year at this point. I've got a small setup in my bedroom with a DDJ-400 and I've been practicing things like beat alignment, timing of starting/stopping tracks, and experimenting with different kinds of transitions. I've learned basically everything I know from YouTube tutorials and watching live sets on YouTube, but one thing that those have never taught me is why you need to use headphones.

My setup is complicated to explain but basically it doesn't let me use headphones. They've never worked for me because I have to route the audio into speakers connected to my computer, which means I have to change Rekordbox's preferences when I launch it to tell it to not use the DDJ-400 as an audio output. The audio works fine when I do this but as a side effect, no audio comes out of the headphones jack in the unit.

Now I'm not saying I'm right or anyone else is wrong, but I've learned how to do what I do without using headphones at all. I line up the tracks visually looking at them in Rekordbox and just fade tracks based on that. It may help that I know my music really well, but I assume all DJs do? So I don't really get it. I must be doing something wrong or missing out on a huge part of DJing but I just can't tell what I'm missing out on because it seems like I can do everything just fine without headphones.