The "open magic in the modern world" novels tend to break down into two types. One in which the magic has resulted in considerably less modern society, with sometimes even feudal loyalties among the canonical werewolves and vampires. Or even among the other races that can be used. Which tends to mitigate the setting.
The other one in which the world is menaced by something from another dimension. Devils out of Hell. Cthulhu-like beings. Or anything else that can be a peril. Which tends to limit the plotlines. Unsurprisingly. A magical/technological society has strict limits on how you can make the magic vital to the story. And magic which is not vital to the story is superfluous.
Putting it all behind the masquerade lets you have a full fledged modern society with more plot possibilities.
Not that that is an excuse to not justify it.