There are a couple of reasons for this. First of all, the way you notate rhythms is helpful for the performer who is reading them. If you follow standard rules of notation, performers will more easily be able to read and interpret what is written, and it makes more visual sense to see beams organized based on the metric accents of the meter. For example, a general rule is that in 4/4, you should tie beats 2 & 3 together instead of using a half note so that beat 3, which holds the secondary metric accent in 4/4, is not obscured. If you're writing in a duple meter (2/4, 3/4, 4/4) eighth notes should be beamed in groups of 2 or 4 but never in groups of three, as this would imply a compound meter (3/8, 6/8, etc.). Even if you have three eighth notes in succession followed by an eighth rest, the beam between the second and third eighth notes should be broken.
Second, there will be times when the metric accent of the piece will differ from the natural accent pattern of the meter. Instead of changing the time signature for a single measure, you can rebeam and add accent marks to notate the unique metric pattern. For example, if you're writing in 3/4 but want to have one measure with a 6/8 feel, you can beam your eighth notes in groups of three in order to imply the metric division without changing the time. You may also use beaming to call attention to non-metrically accented notes that you want to give special musical attention to. As stated before, the performer should be able to recognize these rhythmic patters and interpret accordingly.