They help you write efficient code and solve problems in optimal or near-optimal ways. Without them, you will be reinventing the wheel - not always successfully.
Also, they help you structure your code, so that it can be maintained more easily by encouraging a better design / implementation.
Algorithms and data structures are the basic tools of a programmer. They are as essential as a hammer (or nail gun) to a house framer. They are the tools that solve problems so you don't have to reinvent the solution.
You should understand what they are, why and how they work, and what their shortcomings are. Knowing this will save you a huge amount of time that could be wasted trying to solve a problem that has a solution.
You can get by programming without being proficient in a particular language, but you cannot program without the knowledge of data structures. Data structures are more of a Computer Science obsession. Each problem will have its own ideal data structure that fits naturally to it and to manipualte the data in the structure you will need algorithms.