They’re useful if you want to get some carbs onboard fast. For example, on a trainer ride first thing before you’ve digested your breakfast, smash a gel and have the glucose in your system by the time the warm up’s finished.
Generally they’re more convenient to get carbs in on a whim. If you feel terrible during a session, you may not want to get off the bike, make up some home made carb mix (usually involves boiling the kettle to disolve sugars into water properly). Keep a couple by the trainer to guzzle when needed.
Also for longer outdoor rides, I generally take 1 or 2 in my jersey to hold off any pre-bonk vibes. I did a 4.5 hour ride last weekend and on the hour mark I had real food for the first 2, and gels for the second 2 as well as 2 bottles with 90g of carbs, which I sipped throughout. The beauty is, if you don’t need a gel, you don’t have to throw it away!
If you’re training hard enough, then I wouldn’t worry about weight - these things take care of themselves IMO. On my endurance ride I burned over 4000 kjs, so no way I could even physically eat enough food that day to keep the cals out of deficit.
For me, there’s no difference between a gel and carb mix if you consume the same amount of carbs out of it, so notwithstanding the above, choose your poison!