While plants need all sorts of nutrients to grow, the ones that are in shortest supply are usually nitrogen (N), phosphorus (P) and potassium (K). That's why bags of fertilizer have three numbers on the front such as 5-10-10 (5% nitrogen, 10% phosphorus, 10% potassium by weight).
Nitrogen nourishes leaf growth, the other two are needed for stem, flower and fruit growth. And since nitrogen is generally in short supply in spring, it's a good idea to start off with a bit of high nitrogen fertilizer like fish emulsion.
And organic fertilizers, like fish emulsion, also have other elements and compounds in smaller amounts, always a good thing.