I think it highly depends on the individual. Some people need that deadline to set a fire underneath them that gets them going. Some people face health complications, others might have an event, or maybe just a bet or competition.
As long as one doesn't resort to unhealthy habits or discount the work they've done (i.e. "I only lost 40/50lbs, I'm a failure, time to give up), then deadlines can be beneficial.
They weren't for me, so I stopped using them.
Deadlines might not just work for you. There's nothing wrong with that. Find out what makes you get going and run with it (perhaps quite literally

)