Hi there,
Was thinking about this the other day as I do a little crossfit.
In some “WOD’s” the use of a weight vest is optional for those that want to push / challenge themselves over & above the workout.
Does this have merit for indoor bike training ?
Just pedal harder. And if you’re indoors, weight doesn’t matter.
The comparable method to weight vests for bodyweight movements in cross-fit, is probably low cadence work on the bike.
Probably purely physiological, but I stuff approx 5Kg in my camel back for all my outside training rides in the hope it strengthens my core, shoulders, etc (I’m a mountain biker)
For any race / important ride I take it out and it feels like I’m flying 
This is pure “bro science” though 
Depends what you’re training i guess. If you’re training ve watts, you’ll just go slower (unless you’re going downhill).
Putting on a 5kg vest, might make a slight difference over time if you use it for hard sprints uphill.
If you’re losing weight, you Can use it to see if you’re getting fitter or just lighter (works very Well for running as Well).
I thought they’ve done studies and shown that running with a weighted vest does not make you faster (or at least no faster than running the same mileage without the vest).
Crossfit may be a bit different since the workouts are prescribed 2x20 of X movement…and if you do it long enough with just your own bodyweight, it becomes progressively easier. On a bike we can shift though to create/reduce resistance…the derailleur is our weight vest.
Watts are watts and power is power. Maybe if the added weight caused you to push more power and train harder (without overtraining) it would be beneficial.