Mainly because it is much more convenient and fast. You just pop it in, pee, rinse it off and wash your hands right there.
If you were peeing in a toilet you would have to aim, check if your weenie’s stuck in sprinkler mode, and walk to the sink afterwards. Imagine what you could’ve spent those extra 4 seconds on.
Another great thing is that it saves plenty of water! You use 4.5 - 11L per flush where I live. Instead of wasting all that water you could just be doing a quick rinse, plus washing your hands cleans the sink too.
For the love of god, just sit to pee, your cock won’t fall off and women can’t tell. I promise.
Word!
And, you can be doing other things while you drain the main vein, if you want to. Not suggesting anyone has to forego the pure pleasure of pissing in the name of productivity, but you can if you want to.
Easier when you’re barely awake and wobbly because insomnia as well.
I only do that when I have a boner because it uses more water. Honestly though it’s just more fun to pee in the sink. Also sitting down risks masturbating unfortunately.