Much of the scientific literature in psychology is concerned with proposing and empirically testing theories. However, if you are a practitioner you are interested in how you can apply these ideas to achieve applied goals. This requires that you understand the support for various scientific ideas and that you are able to apply them to a specific situation.
Take for example the Job Characteristics Model. This model suggests that five core job characteristics (i.e., skill variety, task identity, task significance, autonomy, and feedback) tend to increase motivation (among other outcomes and mediating processes).
Thus, you could apply such a theory to make decisions about career choices. For example, all else being equal you might evaluate career choices by the degree to which they allow for autonomy, use of meaningful skill, and so on. Or you could use the ideas to think about how you could modify your position to make it more motivating.
Or take another example, of Holland's Codes which provides a scheme for understanding individual differences in vocational preference. You could use this to think about what particular jobs you might find more motivating.
And of course there are many theories directly concerned with understanding motivational processes.