It's nice to NOT have the hubs have to lock/UNlock/RElock when you have to change direction when in 4wd. It also greatly reduces the chance of failure from said directional changes. Improved MPG doesn't hurt, either. (1-3MPG, depending on factors such as vehicle weight, aerodynamics/drag), along with reduced wear of front axle components.
Even though this question is 5 years old, it seems that it was not fully answered.
Does it make the front end "posi trac?" or something?
The manual locking hubs don't turn the front end into a posi-trac (LSD). They act much like a transmission's clutch by connecting the rotating axle to the wheel so it can be powered.
The differential itself will determine if it's an open diff, an LSD, or a locker.
Whether auto-locking or manual, unlocking the axles lets the wheels spin independent of the axles while in 2WD mode, which helps save fuel by not forcing the front differential to be turned by the wheels.
What are the benefits of locking your hubs manually?
As @KyTerra mentions, when you need to rock the vehicle, auto-locking hubs will disconnect when the vehicle's direction changes and then re-lock in the new direction. This adds a lot of unwanted wear on the locking mechanisms and could actually result in failure if you're applying too much power when they're trying to engage. This is where manual lockers are more favorable, since they stay engaged at all times until disengaged manually.
There are some who claim that manual lockers are stronger and some who claim that our auto-lockers are quite robust. I have no idea and therefore can't comment on any particular style or brand.
Something else to think about, speaking of being robust- Which do you think would be more likely to fail under stress, plastic (nylon) or metal (normally steel for the "driven" parts? I have had the "driven' plastic ring fail on flat ground, while pulling another vehicle into the shop to work on it. Switched to manual locking hubs, ZERO failures. (327,384 miles on a Suzuki Sidekick- hubs replaced at around 170K miles, I think. Yes, it is now almost dead and smokes constantly from oil consumption.)
Since the part that pushes the locking ring to engage the wheel and axle is not actually part of what transfers the power, I would think that a well-made part would suffice. That said, I would prefer a metal piece over a plastic, but nowadays, plastics have been engineered so well that it's possible for them to be plenty robust.
I would still pick a metal component over plastic if given the choice.