'Sweets are the uses of adversity' means what?????????

Well first, this phrase is spoken in II.i, not I.i, by Duke Senior. But it can be related to Act I, which is full of adversity between the parties. For instance, Orlando is terrorized by his brother - a strict adversity - but by manipulating that oppression, he gains the chance to go out with a bit of his inheritance to discover his own identity (the rest of the play tells us his plan is more myriad in scope, but it's what we know in Act I). Likewise, when Charles tells Oliver what Orlando intends, Oliver sees that if he plays the scheme right, he can gain further control of his brother. The theme is that even in a situation rife with adversity and negativity, there is always a "jewel" that can be exploited for the benefit of him otherwise the brunt of that adversity.