Here's why:
* Latias is not available in SoulSilver: The only legendary Pokémon available in SoulSilver are Latios (which you catch using the Soul Dew) and the three legendary birds (Articuno, Zapdos, Moltres).
* Soul Dew is only used for Latios: The Soul Dew is specifically used to attract and catch Latios in SoulSilver.
* Latias is in Pokemon Omega Ruby/Alpha Sapphire: Latias, along with Latios, is available in the remakes of Ruby and Sapphire, Omega Ruby and Alpha Sapphire. You can catch them in these games using the Eon Flute.
So, while Soul Dew is an important item for catching legendary Pokémon in SoulSilver, it won't help you find Latias.