
The Stop Watch - in terms of being a logical weapon to be used against vampires and the forces of darkness, it is the odd-man out. Axe and knife? Cutting and melee weaponry. Holy Water, Bible and Cross? Staple holy relics against vampires. (the Cross having the duel effect of being a holy symbol *and* a boomerang) Garlic is a well-known potent ward against vampires, and even the laurel has history in the bible for being associated with peace and the resurrection of Christ.
So where the heck does the stop watch come into play? Surely when grouped together with the aforementioned weapons in a vampire hunter's arsenal, it would cause anyone else to think it a useless and insane addition. How is a common stop watch going to defeat the demonic hordes of Dracula? It has no holy connotations and certainly no use as a weapon.
In the Castlevania Universe, the Stop Watch has been used as a tool to slow down or even stop the flow of time temporarily in a specific area without affecting the user. This, obviously, increases its use as an effective part of a hunter's arsenal dramatically - making it possible to slow down or stop enemies and traps altogether, therefore allowing the user to bypass obstacles or inflict increased damage without having to worry about constant attack.
However, only the Belmonts and their allies had the ability to influence time and use it as an effective weapon against enemies. (with the exception of time-manipulative demons - something I'll get to in a moment) The stop watch relic was simply a tool they almost always had. Right? But how were they able to use a stop watch in such a fashion, and nobody else was? Did they have something others didn't?
In Curse of Darkness, we are introduced to Saint Germaine - a time traveler who has been keeping tabs on the events of the Belmont clan's fight against Dracula. In his appearances through the game, he talks in an ambiguous way of destiny and how events are to unfold for the proper future to come about. He is later referenced by Aeon in Judgment as being a member of a group of time-travelers. Both characters directly interfere with the events in history with the implied intent of keeping the timestream as it should be. With that being said, it can be theorized that this group have marked interest in the Belmonts and those associated with them, more than likely because of the heavy influence they have in history.
That being said, my theory is that these same time-travelers (maybe even Germaine or Aeon themselves) were directly or indirectly the ones that gifted the Belmonts with a Stop Watch (or perhaps more than one) that had the ability to influence timeflow.
But why? Dracula doesn't have the ability to influence time, nor do any vampires we have encountered yet. In the Vampire Hunter D novels (something I hold so close to the Castlevania universe that they could practically be the same if different events occurred), it is said that vampires never had the ability to master time. They could manipulate space through science and magic, but mastery over time was something they could never achieve. (perhaps due to it being the domain of the time-travelers) Thus far, this same thing could hold true to the Castlevania side of things. However, Dracula and his sorcerers could summon other creatures and demons who DID have time-manipulating abilities, and that would be something the Belmont's couldn't counter against by themselves.
Therefore, we have the subtle interference of the time-travelers, and the appearance of the stop watch as we know it in the games. Is it coincidence that Trevor was the first Belmont chronologically to have the stop watch as a part of his arsenal in Dracula's Curse, then 3 years later we have Saint Germaine pop up? Maybe, but considering that's the first and only time we've seen him in hundreds of years and the two events happen with 3 years of each other, it's certainly not out of the question.
So, in the interest of not overloading anyone with text-poisoning, I'll end it with that. I encourage (intelligent) discussion on the matter. Chances are, this won't be my only over-reaching theory on matters of the Castlevania-verse.