Gives you a strong sense of affirmation (which is no small thing in a sport defined by failure)
Gives you something to crow about to others (how much of an advantage this counts as depends on your personality and situation)
Relieves one major source of stress -- uncertainty. Which can be a very major source of stress.
It's a one way street.
Teams that might otherwise be interested in you, will stop thinking about you as an option. Which may or may not be a big deal, depending on....
... The coach who got you to commit early and his situation.
Because he does not thereby commit to you. No matter who he is, he is likely to keep looking, since he has to hedge his bets.
If he finds a better fit, he may drop you like a hot potato without hesitation.
That particular coach may not even be around when time comes to make an actual binding offer.
Though one major source of stress (uncertainty) may be relieved, another source of stress, probably lesser, is introduced since you still have to do your part to make sure that the "offer" comes to ultimate fruition in the form of a signed NLI.