It is useful to have a goal in mind at the outset, whether it be simply to learn for one's own enjoyment, to be able to perform confidently to an audience, or to reach a certain level. Whatever you decide it is often useful also to have certain specific, short-term aims along the way to provide incentive.
In many ways, success and enjoyment are rather akin to staying healthy and eating a balanced diet! Preparing for grade exams is a good way to develop all-round performance skills and confidence. At least by following a curriculum of this kind, whether or not you actually sit exams, you will be progressing in technique and musicality, with a range of disciplined exercises and varied repertoire.
Acquiring technical skills may be challenging at times and are, of course, not to be seen as an end in themselves. However, perseverance does lead to increasing musical freedom, such as the production of a more beautiful tone, or the ability to play intricate passages with more precision, wherein lies performing character and pleasure. Basic scales and arpeggios, along with other finger exercises and studies, generally form the building blocks of all the melody and harmony you are likely to encounter within your repertoire, and so becoming familiar with them in all the different keys will promote a knowledge of effective fingering, making the learning of pieces substantially easier. Conquering the technical demands of the music leaves the performer free to engage with the emotional expression and communicate it fluently to the listener.