There are many reasons to learn how to divide against into syllables. Separating a word like against into syllables is mainly to make it easier to read and pronounce. The syllable is the smallest sound unit in a word, and the separation of the against into syllables allows speakers to better segment and emphasize each sound unit.
Knowing how to separate against into syllables can be especially useful for those learning to read and write, because it helps them understand and pronounce against more accurately. Furthermore, separating against into syllables can also be useful in teaching grammar and spelling, as it allows students to more easily understand and apply the rules of accentuation and syllable division.
In the case of the word against, we find that when separating into syllables the resulting number of syllables is 1. With this in mind, it's much easier to learn how to pronounce against, as we can focus on perfecting the syllabic pronunciation before trying to pronounce against in full or within a sentence. Likewise, this breakdown of against into syllables makes it easier for us to remember how to write it.